UK retailers generally don't rip off their customers compared to the US, but there is one exception. When a product is newly released, as is the lens the OP is talking about, UK retailers do tend to charge high prices in order to extract the maximum possible from the must-have-it-now-at-any-price brigade. For some reason, retailers in the US don't do that, and I don't know why. But we should all be grateful that it does happen here, because those mugs are subsidising the rest of us.
If you look at products which are not newly released, the economics are quite different.
For example, since the OP is after a Sigma lens, I picked at random the 24mm f/1.4 Art. (Canon fit since I'm a Canon user.) At B&H in New York that's currently $749, which is £563 at today's exchange rate of £1=$1.33. But then if you wanted to buy it there and import it here, you'd have to pay 6.9% import duty, and then 20% VAT, and the full price would be £722. Meanwhile most UK retailers will sell it to you for £649, a few have it at £629, and currently Wilkinson Cameras have it at £599.
Many people like to moan about "rip-off Britain", but the reality is much more nuanced. In the photographic sector, UK retailers are generally cheaper than US retailers when you do the comparisons properly (except for newly-release items as explained above).