Thanks all, very much indeed. I agree that being in the UK and the huge cost are going to be drawbacks!
@Peckish Thanks for the insight into difficulty of rulers and horizontal quilting on a high-end longarm... I think I can probably discount a LA as a solution for now - it's not that it can't be done or wouldn't help (channel lock definitely would help with vertical/horizontal lines) but that the huge investment coupled with the fact that it won't solve my diagonal problems I think means I'll be as well buying and trying the Juki TL-2200 or the newer TL-2300 (the TL-18 is available only in the States I think.)
There never is a perfect solution, is there :-D!