Why not just go to Schneider, they do this as standard, the cost is c £700 per month, but it also includes your telephone costs. They already have a room full of 'like minded' sheep.. erm, sorry, traders......
Alternatively there is also KYTE Group who do a similar thing in Islington.
Apporoach either as a group they may well cut you a deal on a slightly cheaper rate - everything is negotiable!
Either way you get the desk without the risk of signing your own lease which could be very expensive if you don't fill the seats - of course you may be trying to follow STA as a business model so that the other traders pay your way.............
TBS
(nothing to do with Schneider, do use Kyte as a broker)