I wish I could point you at a range of UK IB driver dealers but it's not that simple. Since a number of UK/Euro residents will probably be wondering about IB drivers I will relate what I know.
Steaming Audio of Sweden sell the 15" Tempest X.
Nice driver with excellent Xmax. In a small room you might get away with only two.
They obviously speak English but I have had no dealings with them so cannot comment on their service. At least there would be no VAT to pay because Sweden is in the EU. Freight charges might be quite expensive due to the weight of big drivers. They list their prices in Euro. Conversion websites are readily available online to work out what it will cost you all together in UK dead squid. The Euro is presently almost on a par with the UK dead squid for a rough calculation.
So 4 x Tempest X 15" = ~£800 plus freight. Ouch?
The upside: A four x 15" IB will completely trash any subwoofer you can probably afford and wont be visible in the room. Add the price of an EP2500 power amp and you won't ever stop grinning.
MCAudio (Marshall Choong?) of London sell the Acoustic Elegance drivers. They do not list the AEIB 15 on their website.
I bought my original 4 x AEIB15s via MCAudio and would never have any further dealings with them. A very negative experience in every imaginable way. If you live nearby and can examine the product carefully, pay and then take them away, then fine.
Dealing direct with AE in the US is probably a better bet but I understand AE is presently out of stock of 15" baskets and may not have any until May! AE may not tell you this when you place your order!
Be very aware that John at AE is not a reliable communicator! His products are excellent but let down too often by his poor customer relation habits. I have been contacted by a couple of Cult members who have also had communication problems with AE after placing an order. Doing business doesn't have to be like this!
Fi Caraudio drivers are available in Europe but I cannot remember whether those who bought them ordered direct. Search AV forums for 18" IB threads for more information.
Cerwin Vega Vmax 15 subwoofer drivers are occasionally very affordable (~£100) on special offer from a car spares discount chain in Denmark. (THansens) It seems they are having a clearance sale on them at ~£90.
Generally speaking buying from the US is usually straightforward enough. You may have to collect from your nearest airport unless you pay extra up front for door to door service. You will have to pay VAT on top of the US price plus freight charges plus customs charges plus any additional paperwork customs clearance charges. There is no legal way to avoid this. The dollar has risen from its very affordable 2:1 to rather expensive for UK (dead squid) purchasers.