Hi Jack,
Welcome to T2W.
It would be wrong for me to say categorically that 'scalping' with a SB firm can't be done but, with little doubt, it's going to make a tough job tougher still. There's truck loads of threads on SB firms on here and, if you're seriously thinking of giving this a go, it would pay you to set aside an afternoon and read around the topic thoroughly. Check out the relevant threads in the Trading FAQ forum while you're about it. The key point to keep in mind when using a SB firm is that you're not trading the actual share or futures contract etc. You're trading something that the SB firm's own computer algorythms have generated. Technically, a spread bet is not even a true derivative product, such as the ES e-mini future which is a derivative of the S&P 500 equity index. Broadly speaking, the later will always move in tandem with the underlying, while the former doesn't have to. That's because it's a construct created by the SB firms themselves. I'm not going to get into all the usual circular arguments about what happens when the clients of SB actually manage to make money scalping, as you can read about then ad nauseum on here and elsewhere. All I would say is to put yourself in the shoes of the SB firm. If you had a client who consistently makes money and you don't have an opportunity to hedge against their trades (because of the speed of the transactions) - how would you feel and what would you do?
IMO, if you think you want to trade this way, you'd be well advised to gravitate to a direct market access broker and trade actual shares or futures etc. That way, if you are good at it and make money consistently, your broker will be your new best friend and your commissions should reduce as your trade size and account builds. You won't have any worries about the person on the other side of your trade because it's me - or the prop trader down the road - or whoever. The important thing is it's not your 'broker'.
Needless to say, if you're just doing this for a bit of fun of an evening, trading a few pence or few quid at most - then you're probably fine to carry on as you are. You might make a bit, lose a bit; either way, it's no skin off anyone's nose.
Good luck with it!
Tim.