Hi Smokestack...
Yep, UK born and bred (well, until a fortnight ago - just moved to Germany and it's proving tough to find a hook up) and very interested in the way our cannabis laws are going back home. I'll be moving back in a little over a year anyway, so it's good that I keep my ear to the ground.
As for your question, I would have thought that if your girlfriend protested ignorance of the whole thing while you backed up her story and confessed to it all, there's no reason why anything should happen to her. Last year at uni we grew skunk while living with a strongly anti-'drugs' (his word) person, but he seemed happy enough for us to guarantee to absolve him of any blame if we were discovered.
I don't know what experiences you've had in general with the police and pot, but in my experience unless you're suspected to be dealing then they really couldn't care less. One problem with the near blanket American perspective on the web is that it becomes easy to forget just how liberal things are in Britain when it comes to personal use. If you have just a few plants in your loft then I honestly can't imagine that (a) anyone's going to find them (unless you live in a terrace, in which case you should use a cupboard or something) and (b) that you'd get anything other than a slap on the wrist anyway, assuming it was your first offence and you weren't growing enough to make selling worthwhile.
I know that this is just my perspective, not hard and fast rules, but perhaps the most important point is to remember what country you're in and that American governmental sensibilities towards these things differ greatly from our own. I don't know a grower who thought twice about keeping plants, and I should imagine that as long as you keep quiet about it you'll be fine...
Hope this helps (re-assures, maybe?) and good luck in your horticultural experiment
I think my e-mail address is on my profile, otherwise try Spiderman420@iFrance.com if you want to chew the fat about pot on our island sometime...
Take care...