I started attending meetups because of a watchlist notice, and we get lots of people every year who come to London meetups because of that notice. We have also had some success in the past with inviting editors who we know are in or near London.
As for donors, Fabian and I ran a couple of training events for donors in the London area a couple of years ago. We got quite a few attendees as I remember, and I think we invited donors to one of the backstage passes. I'm not sure I would invite donors to meetups though, as I can see some cultural differences. Meetups are a great opportunity for experienced editor to share experiences and discuss things with fellow editors, things like Wikipedia's tenth birthday party were a good opportunity to answer questions about the project from donors. But while a meetup is a great place for a five minute lesson on a tool like hotcat, or a 5 minute chat about a specific problem, a noisy pub isn't a suitable venue for an hour long training session for a new editor.
I've given up many an afternoon or evening to train other people, and I might go for a beer afterwards. A London meetup is a more social event where I and most of the others drink beer, and if I knew that I was going to be spending my Sunday afternoon training then I would stick to coffee. So I'd be happy to invite newish editors to meetups, but I prefer to have some events where you can assume that everyone round the table has done some editing.
Regards