In reply to alex:
There are private travel clinics that should be able to get you in sooner than your GP and don't cost all that much. I think the consultation fee was £20 for the one in Sheffield.
I'd prefer to pay a little extra and get the stuff in the UK where you can be (more or less) sure it is real. Malaria is horrible. Of course you could get it in Thailand where it is likely to be ok, but counterfeit stuff is more prevalent than in the UK - depends on what level of risk you are comfortable with.
Alternatively, if you just want a private script and don't need advice (Malarone versus doxy debate, etc.) or immunisations (I assume you have looked in to all this already!), have you tried phoning for any appointment with any GP without telling the receptionist that you want something travel related (I imagine they have a specific travel service at the GP if they are making you have a 20 minute appointment) - might be able to get a normal appointment a bit sooner.
Also, it seems not terribly reasonable to blame the GP practice for being shit if you haven't planned even 2 weeks in advance...