Those places aren't really what most mean by prop houses - also, there are very few in London - Mako Global is one example - those places hire engineering/computer science grads, MSc grads etc and they mostly trade higher frequency options market making strategies.
In London you have a lot more prop shops that are basically for discretionary traders, albeit you are encouraged to fit a certain style, low risk day trading etc.
The difference is pretty massive really, at the salaried places its more like a mini, high tech hedge fund, and most people working there would be programmers and developers, trading stat arb, and options MM strategies - whereas at the prop shops we're all BSD locals, throwing size around and generally smashing it up.