Good venues for group lunches in London?!

Good venues for group lunches in London?

I run a social group for girls in London and try and arrange get togethers to welcome new members as the idea behind it is to make new friends, connect with like-minded girls of a similar age and improve their social lives.
The next get together is only a few weeks away and I am trying to find a venue that will accommodate about 25 people. Preferably a nice bar in a central or riverside location where I'll be able to book an area so we can have nice drinks, maybe something to eat. As it's at lunchtime; somewhere that opens from about 1pm
Can anyone make some recommendations as I'm running out of ideas!