Note: In this, I'm assuming you're talking about the South African War (or "Second Anglo-Boer War," "Tweede Vryheidsoorlog," et cetera) from 1899 to 1902, and not the shorter, sharper conflict with the Transvaal rising in 1880-81 or the battles that attended the annexation of Natalia (1842-43) and the creation of the Boer Republics in 1854 (Free State) and 1852 (Transvaal / South African Republic / ZAR). This is also kind of stream of consciousness, so if something's unclear, just ask.

A great many were, in fact, highly critical of British policy towards the Boers of the Highveld. Their discussion usually centered on the ZAR as the major focus of news and a potential reason for this conflict because of the gold on the Witwatersrand there (Johannesburg). Although the investment capital (money and concessions/profit) was largely in British hands, Boer governance depended on a status of citizen (burgher) as recognized in the Republic, which provided a moralizing casus belli via the demand that white migrant mine workers and other newer immigrants (uitlanders) receive the right to vote and proportional representation in the legislatures (the Volksraad; after 1893, two chambers). Beyond that, the legal question was also deliberately muddied with Britain claiming that they had never really relinquished the ultimate right of sovereignty over the Boer Republics, a point that most European states respected to such a degree that a minor violation of protocol (the Kaiser in Germany sending a congratulatory telegram to President Kruger in 1896) could threaten conflict in Europe. So the question of how "annexationist" people viewed the SA War is not itself entirely clear.

That said, empire was not itself necessarily unpopular, and the British did not see the Boers as being quite like them on the whole. If one asked about the "race issue" in southern Africa before 1902, they'd say "English versus Dutch" or "Briton and Boer." ("Native Question" and "Asiatic Question" were the names given to other sorts of intergroup issues.) Certainly there were many jingoes who were happy at the idea that Britain would put the Boers in their place, like Arthur Conan Doyle, and some of the permanent establishment were quite eager to reverse the "loss" of 1881 that re-created Boer autonomy in the Transvaal. The ascendant regimes in the British government in 1899 (Colonial Secretary Joseph Chamberlain in particular) and at the Cape (Governor and High Commissioner Alfred Milner) were surely pushing for war. In fact, the mood was so publicly hostile that the Boers opted to strike first before British forces could build in the subcontinent. That fact was not especially important to critics, but it appeared in justifications at times.

These critics were sometimes lumped together as Pro-Boers, when in fact they really didn't have anything in common besides being against the war (Bill Nasson talks about this in his The War for South Africa, 2011). W. T. Stead, the famous moral crusader and publisher, was one of the most important in terms of appealing to public morality in the early days of the war; J. A. Hobson wrote a damning discussion of the war's economic motivations, although his analysis had some presumptions about the "loss" or "gain" of the mines that the reality of capital in the mining companies really rendered problematic. Hobson would, in 1902, extend his critique to the entire concept of imperialism (Imperialism: A Study) and so jump-start a major mainstream academic critique of empire that continues to this day. Other critics, like future PM David Lloyd George, would have very important influences on the Empire as a whole.

With the end of the conventional phase of the war in 1900, and the rise of the guerrilla campaigns that lasted until peace in May 1902, the British public became much more skittish--the concentration camps used to deny succor to the forces still in the field, the rampancy of disease, the damage to infrastructure, and the cost of a mass army (not to mention the absence of the people so levied) put the conflict in the public mind in a much more critical way. Emily Hobhouse is of course the crucial publicizer of British failures connected to the condition of internees and prisoners, but she was not the only one, and her commentary resonated to the point that Parliament in London made many inquiries about it. Some former pro-war figures also expressed distaste with the 1900-1902 phase, and became more critical of imperial adventure. This was different to them, even if the Boers were "degenerate Europeans," they weren't Africans or Asians whose voices could be silenced by imperial power structures and who could simply be run roughshod over. What's more, as time went on, the fellow feeling between loyalist Afrikaners in the British colonies and the suffering Boers grew a great deal, so they were heard in important ways that would have a long shadow and help determine political mobilization after the war. In addition, various figures toured in order to publicize conditions and collect donations, which had an effect on the Continent but did not go without notice in Britain. Of course, nobody spared much concern for non-whites in the future South Africa, which indicates the limits of public debate to being around the Boers even though much worse was, and would be, visited upon black populations, including concentration camp internment.

On the so-called "pro-Boers," see Eliza Riedi's "The Women Pro-Boers" in Historical Research 83 (2013), 92-115; Ken Morgan, "Lloyd George, Kier Hardie, and the Importance of the 'Pro-Boers'," South African Historical Journal 41 (1999), 290-311; Stephen Koss's very old but still helpful The Pro-Boers: The Anatomy of an Antiwar Movement (Oxford: Blackwell, 1973); Elizabeth van Heyningen, The Concentration Camps of the Anglo-Boer War (Johannesburg: Jacana, 2014), and perhaps Nasson's The War for South Africa which I mentioned above. Most of this above comes from a variety of publications at the anniversary of the war, but it is repeated in those publications.