For what it's worth because this looks like it has been done to death ! A good friend of mine works at a larger SB firm for obvious reasons I'm not going to name him or his company.
Without giving too much away they have been discussed in this thread and are one of the bigger players.
He CATEGORICALLY states that " clients trading short term (sub 5 minute) at larger size £50+ a point who consistently win will be referred to dealer, they are a business and want profitable business and they don't pay his wages by giving cash away "
He also confirms that they watch for clients who have a fast feed and an even faster finger, he reckons this activity is really watched for and anyone doing it will last no more than 48 hours before they're put on dealer referal.
It's not all bad. If you trade over longer time periods and consistently win his SB firm love's you. When they hedge your position they put in some of their own dough (because you are so good and win all the time) and they win too !
If anyone wants free money I would suggest Worldspreads. 2 point dow spread, £500 losses refunded (i think) and prices so slow you could post your trade confirmation and I bet the prices would have caught up just as the postman pops your envelope through the letter box of world spreads towers in Regent Street.