Britains main underlying reason for a lot of its actions was trade. India wasnt conquered for fun or power, but profit, it was conquered to ensure trade was continued, and more importantly, the French didnt cut off the trade for themselves. Thus conquest for power was less of a concern than conquest for trade (altho reasons could vary somewhat over time). The reason that places like America were replacement-colonised was because the locals lacked the socioeconomic structures to trade efficiently, far easier to colonise the place with Brits and then trade with them.

Britains decline in colonial power occurred at a time of relative stability, so trade wasnt 'cut off' as such, but became subject to standard free market pressures . Additionally changes in economic circumstances meant that direct trade was less important by the mid 20thc, if your main source of wealth is from making high end products to then export and sell then cheap primary products are important, post WW2 though Britains role as major manafacturer was declining rapidly (and had been for a while), so cheap imports became a less important element. Britain was 'lucky' that its Empire declined when it did, that is has been able to largely reinvent its economy, and that we live in stable times, there are no barbarian hordes lurking on the horizon (and theyre going to have a referendum to leave in a few days anyway).

On the other hand declines can take a long long time to manifest, we dont know where they'll be in 100 or 200 years time.