How about:

Metro Life PA
A-Z PA Services

As you can see with my business name I like having my location in there as I initially am targeting my local community. London is a little different as it's so large.

I doubt you're targeting a particular part of London though. And from what I hear a lot of businesses in London outsource to India for VA services as it's more cost effective. I would think you would need a name that would really stand out locally.

Good luck,
Janine
Whistler Virtual Assistant