Hi Simon,
Trading with a DMA broker is a good idea. However, to the best of my knowledge, in order to be able to trade on very tight or, within the spreads, the account deposit requirement is a minimum of $20,000.
Not all SB companies are bucket shops. I've been with GFT for a year, been requoted once and had most of my orders filled without slippage.. At times, their spread, on GU, will be 2 pips (usually 3), which I believe, is very comparable to a DMA broker.
There are some rip-off SB companies out there. Unfortunately, some of the biggest names are the worst culprits.