Hi Anna
It sounds like she is very unsure of strange dogs, for whatever reason. For this reason, although it is good that she has doggy friends, you also need to practise with strange dogs around. And not just one or two, because then they will be a novelty. You need something like a parade of strange dogs to go past her, so that eventually she realises (becomes habituated) to the fact that they are not a threat.
The strange dogs should be at a distance she is comfortable with, and that she shows no reaction at, to begin with. You should click and treat that "comfortableness". Repeat many times.
Then they would parade by at a slightly closer distance. If she remains comfortable, click and treat. Repeat many times.
The rules might be hard for you to follow if you live in London though - because while you're going through this slow desensitisation to other dogs, you should not let another strange dog get closer to her than the distance you have previously trained her to be comfortable at. Does that make any sense? For example, if she is fine with lots of strange dogs at about 20 metres away, walking past her, and then you walk on a pavement and come across a dog 2 metres away, which she is scared or aggressive of - then you will have undone all your good work.
Plus it's not exactly easy to set up parades of dogs. One way might be to take her to some open shows, if they are held outside in a spacious country-ish area - enter her, so that you are "allowed" into the show area, but don't go in the ring, obviously. Then you can spend the day standing at very slowly decreasing distances away from the rings, observing the other dogs, and clicking and treating constantly for calm, unafraid, unaggressive behaviour.
By the way, I would literally carry a whole load of incredibly tasty treats, and if you meet a dog and she is fine with it and not aggressive (or afraid), click, draw her away slightly by the lead and give her LOADS of treats. Remember to draw away from the other dog's reach or you could have a food fight on your hands!
Another suggestion is to take her on one of the Kay Lawrence courses - clicker and positive reinforcement only, although no prior experience necessary. Kay has a course specifically for dogs which have problems relating to other dogs. Not necessarily aggressive, but possibly afraid or undersocialised etc. It is called the "Progressive Social Development" course, and is held over one-day, 9-5. I've just checked her website to give you a link to it, but they've changed a few things round and I can't see it listed there anymore, so you might want to email them and ask about it.
Here is the link to their website: <a class='url' href='
http://www.learningaboutdogs.com/acatalog/shop.html'>http://www.learningaboutdogs.com/acatalog/shop.html</a>