It's quite possible to find ISO keycaps (like this one: http://www.keyboardco.com/product/double-shot-filco-105-key-uk-keyset.asp), but it's easier to find ANSI sets.
Practically any MX compatible set will fit the WASD since it has a standard layout and as others have said, it's a good board.
I think a Filco would actually be cheaper for you (since you can get them in UK without having to order from the States) and they're even better than WASD. This is a great company for mechanical boards in UK: http://www.keyboardco.com/index.asp
And they have boards and keycap sets in many layouts.
May I ask, what design / colours you really want on the keycaps? There may be a set we can recommend.
I second that. You are in the UK, so you have one of the best keyboard reseller very close to where you live: the Keyboard Company.
I would advise you to get a Filco and maybe even a TenKeyLess with a separate numpad if you need one. Not that the WASD is not good, but having the keyboard shop nearby is an advantage, and both Filco and the Keyboard Company have a very solid reputation. I think you can even see and try the keyboards in person if you drive there.
The doubleshots keycaps Oobly has linked to are very well made. They are a little taller than the keycaps that comes with the Filco, and heavier too. This give a very solid, kind of retro look. They are made by Filco and fit perfectly on their keyboards, full-size or TKL. I purchased several sets in AZERTY.
A Filco with these keycaps is a keyboard that can last years, probably decades.
I would recommend you to purchase O-rings: these are very durable rubber rings that you put under the keycaps, and they dampen the sound of bottoming out when you type. They are not expensive, and you may find that you prefer the keyboard to be more silent. It's good to have them just in case.
About the ISO layout: I don't think you need to convert to ANSI unless you really want a very large choice of keycaps. There are some nice keycaps sets in UK ISO available for Cherry MX keyboards that will fit perfectly on the WASD or on a Filco. Personally, I did convert to ANSI. I'm french and we normally use AZERTY on ISO, but I use AZERTY on ANSI. I prefer the larger left Shift and horizontal Enter key on ANSI keyboards, but that's just me. If you are fine with the ISO layout, you will find keyboards and keycaps sets for UK ISO.