Slavery and colonialism benefited the British population at large in that it was the foundation for the development and advancement of their society. Their wealth, their infrastructure, their advancements were all the result of their subjugation of others.
How do you know this? All the evidence I've found in Economic History (my specialism) so far is that England was already seeing income growth (well, stagnation before an incredibly slow growth for London and SE, as opposed to Malthusian falls experienced in Europe) above its European neighbours for 150 years before 1562, when the first English slaving expedition started. This is why the question of whether it was essential for (capitalistic) development is not clear cut. I still think it was an integral part in England's rise, specifically because of how it changed the power dynamics in Britain towards a merchant elite, but the question is not clear cut. We probably won't ever know for sure - we're dealing with counter-factuals.
Have you ever read the book Capitalism and Slavery by Dr Eric Williams?
To my discredit I havent but will be taking that out of the library the moment I touch down in Port of Spain. Trying to find Trinidadian history books in this country is surprisingly hard even with an academic library. The problem is that it was written in 1944, and our knowledge of slavery and the economics of colonialism has progressed quite well. From the 60's onwards Economic history started using quantitative methods quite intensively, which revealed things like a much smaller profit from slavery than previously thought. Rather than highlight papers you won't be able to access without a university pass, this book review from 2002 does a good job of highlighting the major issues in the field;
http://www.history.ac.uk/reviews/review/259The problem is that we imagine Britain the nation siphoning off cheap resources and thus benefiting from lower prices, but tat's not the essence of capitalism here. What happened was that the 'benefits' of exploitation were captured by this powerful mercantile elite, who demanded prices higher than the world price for the goods made in the colonies, whilst simultaneously pushing for policies that heavily favoured these products whilst discouraging/embargoing alternative sources from outside the British Empire. This gave them the capital needed to invest in the machinery and land necessary for the Industrial Revolution.
However, the question on whether this was
essential or [greatly accelerated[/b] British Industrialisation is very contentious simply because there are so many factors at play. Britain already had more constraints on the Monarchy than the other big colonial powers (France, Spain and Portugal) before 1400 (Magna Carta being important here), which some see as the truly critical step for industrialisation, as it allowed investors and individuals to retain the proceeds of their work whilst discouraging monarchs from racking up huge debts (which undermined banks when they went bankrupt and refused to pay). Secondly, Britain had an abundance of natural resources that would be important during the IR, namely coal and iron. Thirdly, the restraints on the monarch meant that they could not capture all the benefits of colonialism like what happened in Spain, Portugal, and France. This left individuals with capital to reinvest, expanding the mercantile sectors. Fourthly, Britain had a strong legacy of seafaring unmatched by the other European powers as it was essential to their defence (the "wooden wall") - it's no accident that Portugal was the first on the scene (given they also had a pressing need for a navy). They were lucky that navies were going to be so essential from the 1500s to 1950s in that regard.
The other critiques of it being essential point to the lack of progress in Spain, Portugal and France. The former two certainly profited monetarily more than England simply due to the amount of rare metals and gems they expropriated from Latin America. This caused massive inflation locally and across Europe for a hundred years, so it certainly wasn't all that beneficial in the end - it hurt domestic industries as the currency appreciated (lowering demand for exports) and really hurt teh poor, whose costs of living rose as a result.
If you find the question of why nations are rich today, definitely check out "The Wealth and Poverty of Nations" by David Landes, it's a good read. Also check the work of Nicholas Craft, who changed the way we thought about hte Industrial Revolution (turns out British economic growth took place later than we though - after 1850 for "modern" rates of growth, as Steam was totally cost-inefficient until the invention of high-pressure steam), and Stephen Broadberry, who has done a bunch of papers on the "Great Divergence", i.e. when did Europe pull away from Asia in development?