For direct access either IG, GNI or E-Trade. Test out the platforms to find out which you like best then get negotiating, don't pay more than 8bps. Laugh at them when they quote you 20bps.
If you find that you trade FTSE 100's and are always lifting offers and hitting bids then consider E-TRades flat fee structure. It's not DMA and their are certain limitaions, but at £9.95 per ticket it's a good place to start. CMC used to be a good starting point, but since they started charging 8bps for a non DMA product they are a waste of time.
