I have used all the following:

http://www.timothydunnflowers.co.uk/
http://www.paula-pryke-flowers.com/index.html
http://www.alexandralyons.co.uk/

The latter two both have outlets in the West End (Liberty's and John Lewis respectively) so should be able to turn round central deliveries very quickly.

Teleflorist / Interflora are just agents for a whole string of independent florists - while you'll definitely get something acceptable, you may not get something special.