I have plenty of advice for you, but for the right girl you may have to travel
1. Make sure the girl isn't slimmer/taller/better hair/younger or (insert whatever insecurity your girlfirend has). Pick a girl she fancies, not you.
2. Trust me, on no account appear excited or pushy for the idea, let her bring it up and discuss it.
3. On the night, you ONLY touch and play with your girlfriend, however tempting the other girl is.
4. Get an English girl who you can talk to and explain exactly what you want (strap on etc), way before the actual night.
5. Plan far ahead, do not wait till the last minute.
6. Afterwards, when chatting about it, focus on what she liked, not what you liked.
My current girlfriend suggested something similar 5 years ago and I played it cool on the 'first try' now we go every few months. I am convinced its because I did not appear to enjoy it 'too much' (allthough I do). Play it smart, and it will not be a once in a lifetime, it will be repeat visits. Eventually girls of your choice and the things you want to do.
Take a peak at some of my reviews for suggestions, although they are Watford rather than Enfield. There are other girls we have seen before I joined here, in the general area, that I am happy to advise on, but this would be over the last few years (not so recent).