Actually the view that Britain acquired a lot of its wealth because of exploitation of its colonies has a lot of evidence against it, from the point of view of economic historians. There's two chief arguments against it, firstly that European colonialism was actually harmful not just to its victims but to the 'home' country's economy, and secondly that all trade wasn't that important to British economic growth, let alone trade with colonies in particular.

Colonisation harms the home country

So firstly: European colonialism as a harm to the 'home' country (I'm going to be very Euro-centric and call the colonising country the 'home' country and its victim the 'subject' country, because trying to keep 'coloniser' and 'colonised' straight gives me a headache). This argument dates back to Adam Smith in the 18th century, in his 1776 book The Wealth of Nations and more recently is championed by the economic historian Deidre McCloskey. The basic insight is that it is better to have a richer trading partner than a poorer one, but colonialism by its nature is destructive to its victims, it is a violent process directly destroying physical capital and its victims lives, and less directly harming social and institutional capital. The colonial officers typically are out to line their own pockets and head home, hopefully before the local diseases killed them. And when the fastest communication was by sailing ship, the ability of the head office of the East India Trading Company, for example, to control said colonial agents was minimal. What's more, even if the colonisers were truly public-spirited, they typically would lack local knowledge of political, cultural and environmental factors. Maddison's long-term historical statistics show that India's GDP per capita actually declined from 1600 to the latter part of the 19th century (and GDP is a measure of what is produced in a country for a given period of time, regardless of whether that product is consumed domestically or exported).

This argument does depend on assuming that the alternative to colonisation is that the subject country would trade with the home country mostly peacefully and freeishly, as the US did with Britain post the American Revolution. This was not inevitable, the strongest argument for the value of the British empire to Britain was that if Britain hadn't built its empire another colonial power would have (presumably France) and then would have cut Britain out of international trade flows, Niall Fergusson is a proponent of this argument. It also is possible that in the absence of European colonialisation more countries would have cut themselves off from trade like Edo-era Japan did. 

O’Rourke, de la Escosura and Daudin (2008) discuss another argument as to why colonialization can harm the home country’s economy: in the case of Spain and Portugal the immense amount of gold and silver flowing in from their American colonies meant that the Iberian authorities (the monarchy, the aristocracy) didn’t need to build representative institutions to raise taxes. The flow of gold and silver to Portugal and Spain also may have damaged the competitiveness of local industries, a kind of ‘Dutch disease’ (so named when Dutch manufacturing struggled with the development of North Sea oil extraction).

How much was Britain trading with its colonies anyway?

In 1913, imports supplied 25 to 30% of Britain's national income, however this figure is for all imports from all countries, colonies or not, including neighbouring European countries and the USA (a major wheat exporter). You can explore UK imports by country during the 19th century at http://ricardo.medialab.sciences-po.fr/#/country The by country figures themselves are scatty and terribly confusing due to changing names but by continent, the UK was importing far more from Europe and the Americas than from Africa and Asia.

The colonies also didn’t make much contribution to the British government’s budget, it's surprisingly hard to find detailed historical statistics for this in the 19th century, but any contribution was fairly small. There's been some histographic debate over the importance of Britain's colonies to the prosperity of Britain, but no one indicates that colonies made a major contribution of the colonies to the British government revenue. The strongest I've seen is that India paid for its own "defence" (it's a bit weird to describe "keeping India under British control" as defence), and made a small contribution beyond that.  (Gartzke, Rohner, 2011).

The economic historian Deirdre McCloskey (2006) made the following summary:

The cost of protecting the Empire devolved almost entirely on the British people. (A century earlier the British had likewise paid for the defense of the first empire, in what is now the United States; the colonials refused to pay as little as a small tax on tea for imperial defense.) British taxpayers 1877-1948 paid for the half of naval expenditure that was for imperial defense, a by no means negligible part of total British national income each year.

...

Did acquisition of Empire, then, cause spurts in British growth? By no means. Indeed, at the climax of imperial pretension, in the 1890s and 1900s, the growth of British real income per head notably slowed.

Other examples of the relative unimportance of colonies is how France had its boom years, its Trente Glorieuses, post WWII as it was losing its colonies, as did Japan and Italy when they had given up hope of having their own.

How important is trade anyway

Finally, how important is trade to a large country's economy anyway? As I mention above, imports in 1913 supplied about 25-30% of Britain’s income. Which is not be sneered at. But British GDP per capita more than tripled from 1700 to 1913 (see Maddison again). And this was predominantly down to technological change, and a large growing domestic market – British population increased too, doubling over the 19th century. Britain’s terms of trade fell during the Industrial Revolution, despite rising exports, indicating the cost-reducing nature of technological change (in less technical terms, each unit of Britain’s exports brought a falling amount of imports, e.g. Britain was exporting tonnes of iron for fewer and fewer tonnes of American wheat or Argentinian beef, but as iron production efficiency was increasing so much, Britain was still getting rich).

There are arguments that trade was still important to the British Industrial Revolution because the larger markets made British manufacturers more likely to innovate and invest. I'm a bit sceptical about this as British innovation during the Revolution doesn't seem to have been that dependent on scale, inventions were more like one person or a small workshop tinkering with and building on an existing technology, nothing like the scale of R&D going into new technologies like new medicines or self driving cars today. But this awaits a more comprehensive study.

So that's an answer to your question 1.2, I'm afraid I can't answer your questions about the financing of WWI or the loans afterwards. 

Sources

http://ricardo.medialab.sciences-po.fr/#/country

Maddison Project Database, version 2018. Bolt, Jutta, Robert Inklaar, Herman de Jong and Jan Luiten van Zanden (2018), “Rebasing ‘Maddison’: new income comparisons and the shape of long-run economic development”  Maddison Project Working Paper, nr. 10, available for download at www.ggdc.net/maddison .

"Foreign Trade: Competition and the Expanding International Economy, 1820-1914," Chapter 17 in Floud and McCloskey, The Economic History of Britain, 1700-Present (1981), Vol. 2, pp. 50-69; reworked in the second edition; Harley used it in the third edition by Floud and Johnson, eds. http://www.deirdremccloskey.com/docs/graham/britain.pdf

Deidre McCloskey, 2006, Keukentafel Economics and the History of British Imperialism, South African Economic History Review 21 (Sept 2006): 171-176. http://www.deirdremccloskey.com/docs/imperialism.pdf

L.E. Davis, RA Huttenback, 1977, Public expenditure and private profit: budgetary decision in the British Empire, 1860-1912, The American Economic Review, 1977 - JSTOR, https://www.jstor.org/stable/1815917?seq=1#metadata_info_tab_contents

Offer, Avner. 1993. The British Empire, 1870-1914: A Waste of Money? The Economic History Review 46 (2): 215–15. doi:10.2307/2598015.

GARTZKE, ERIK, and DOMINIC ROHNER. 2011. The Political Economy of Imperialism, Decolonization and Development. British Journal of Political Science 41 (3): 525–56.

Kevin H. O’Rourke, Leandro Prados de la Escosura and Guillaume Daudin, 2008, * Trade and Empire, 1700-1870*, TEP Working Paper No. 0208, https://www.tcd.ie/Economics/TEP/2008/TEP0208.pdf