try Chiswick Honda (not that far from you), they normally have a few and they have a website. If you see one and want a fair appraisal before making the trip to chiswick let me know. I live up the road from there.
also, xrv.org.uk, there are a couple in London for sale (some prep'd).
I wouldn't be too concerned with engine size (the difference between a 650 and 1000 isn't really worth £2000+). My AT (750cc) pulled 100mph, loaded with metal panniers, a rucksack, a girlfriend and me. You'll travel slower than you think you will though. 50-60mph is our normal cruising speed while away, things move too fast as it is...
Also, BMW Guildford let you take the demo bikes for a couple of hours. So take you pillion with you and see how wheezy it is. Varied roads round there to test it on.