Thanks very much for the info, Dorothy - that's really helped clear things up for me.
In case it helps anyone else I've plugged in your figures for the Gallop frame to get an approx final bill for illustration purposes (which will depend on the exchange rate):
Which is interesting as it reveals it's $28.41 (£21.78) cheaper to buy it in dollars with the current exchange rate (assuming no conversion fees):
£1,861.84 --> $2,429.25 vs $2,400.84
The Woods Cyclery are listing Sam frames for £2k so I think if I go for one I will just use them seeing as the difference is not huge and those fees sound pretty variable! Plus it feels good to support the UK's only Riv dealer.