It's not as far north, but...
...it's a heck of a lot colder than London:
Don's Cold Hardy Citrus
Granted, they're not looking at grapefruit, but I'm duly impressed with anybody who can get an edible Poncirus hybrid. A very cool site, and a good resource for those interested in breeding cold-hardy citrus.
The site is set up in reverse blog fashion, with the new stuff on the bottom, which I find unreasonably distracting and odd.
(Todd Wert, a friend of mine from grad school, pointed this page out to me).
Also, here's a fun-looking citrus page in German, one of the users' pages from the forum page.