Trying to find the UK customs tax rate on any particular item you buy outside the EU is tough going. I was thinking about buying a used lens from
www.kevincameras.com, simply because they had by far the best selection readily available on line (I didn't buy it, so can't vouch for their service). Eventually I tracked down the customs tax rate as being a few percent - not enough to discourage me, but it was still difficult to be sure I had the right rate. VAT at 20% obviously adds a big sum, but that is predictable. I think in most cases UK buyers would not pay US sales tax, which partly offsets VAT.
In the end my main concern would be if I had to return an item, so, from this side of the water, comments on reputable dealers like B&H are useful, though I don't see the value in buying new photographic stuff from the US for my needs.