Keep in mind that many US brands like Klipsch can be purchased in the UK. You need to weigh the cost of shipping them against the cost of what you can buy in the UK.
As an example, Wharfedale Diamond 10.7 are commonly £579 in the UK, by direct currency conversion, that is US$900. However, they are US$1299/pr from Music Direct in Chicago. That's a $400 difference.
I suspect, though I don't know for sure, if the speakers are used, the import tax and perhaps the VAT may be different than if they are new.
I'm a little confused by your selection of speakers. The Infinity are about US$329/EACH. The Polk Monitor 70 are about US$220/EACH. The Polk Monitor 70 are a discontinued model and as such, are being sold at a very deep discount. I personally didn't find the Martin Logan Motion speakers to be all that impressive, though this was right after auditioning some Focal 836 speakers. But they are running about $249/EACH.
Also note you can get the Monitor 70 directly from Polk Audio's Ebay site of only US$305/PAIR -
[PAIR] Monitor 70 CHERRY Tower Speakers Polk Audio | eBay
That is an outstanding bargain.
In UK speakers, the Wharfedale Diamond 10.5 are about £419/pr (US$654/pr) and the Tannoy Mercury V4 are about £329/pr (US$514/pr).
Now that we have all the models and prices laid out, we can get down to basics. Exactly what is it you plan to do? Are you in the USA and planning to buy speakers you can take back to the UK? Are you planning to buy speakers in the UK to take back to the USA?
Is this stereo or surround sound, and what other equipment do you have?
I think you have to just look at this as speakers, not UK speakers and US speakers. Buy something you like at a price you can afford. Then when it comes time to move back to the place you are currently not at, you can decide what to do with the speakers. Again, there are a lot of details you need to check into. I'm sure the tariffs on used equipment is different. So if you are in the USA, it might make sense to buy speakers there, and ship them back when you are ready to leave. But, I don't think it makes sense to visit the UK, buy UK speakers, and ship them back to the USA then have to eventually take them back to the UK again. I think you'll find unexpected expenses.
Or, given how cheap they are, you could get the Monitor 70's directly from Polk/Ebay, and sell them cheap when you are ready to leave. At $250 each, they might be worth taking with you, but at $300/pair, it is probably easier to sell them. Though they are something of a bargain, the USA Suggested Retail price on them is $919/pr. That's something to consider.
Monitor 70 Product Page : Floorstanding loudspeaker : Polk Audio
The Polk Audio Ebay site also has many other speakers worth considering -
polkaudio | eBay
Just a few thoughts for now.
Steve/bluewizard
