Tradefair (aka Capital Spreads) was doing a £100 opening offer, but the usual answer to the 'how far can I take a small account' question is £0, unfortunately, which is why SB companies can be so generous.
you have to evaluate the offer in the broader context of spreads, margin requirements, reliability, and rumours of stop-hunting.
but an alternative to sign-up offers, is cashbacks on losses in the first few weeks of trading.
eg, if you lose £600 in the first 4 weeks, they will refund the first £200, etc. that kind of thing.
worldspreads have things like that.
remember to read the small-print; it usually says you cant actually take the money out!