Can I withdraw money from my account when a deposit is held against it?

I want to hire a car next weekend to go away with my girlfriend. However the deposit is £250 and I'm not exactly minted at the moment! Lets say I have £300 in my account and I pay the deposit of £250 does that mean that I can only withdraw £50 and the rest is not available to me? Or could I still access the full £300?