Hi Sally, which part of London are you in? Might be able to recommend someone.
A good set of lash extensions is down to the skill of the technician applying them. There's not that much in it between the main brands. So you really want to be looking at pictures of their work as opposed to getting too caught up in what brand they're using.
I mainly put 10-11mm on my clients. So they still look natural, like they're got a really good mascara on. I don't like using the longer lengths because they don't last as long or look as full.