I'm currently working in a school as a cover teacher after graduating with a first-class Psychology Degree last summer, and have been accepted on to the Psychology PGCE course at Manchester Met starting in September.
In my experience, Psychology is becoming increasingly popular at KS5 and a few Secondary schools are starting to do it at GCSE level, which is hopefully going to become more widespread over the next few years. When I was searching for a University to study my PGCE, I found only 7 offer Psychology as a stand-alone subject, however more universities offer Social Science PGCEs.
When I was applying for my PGCE I asked the Head of Social Sciences at my school which would be better to do: the Psychology or Social Sciences PGCE? She advised me that there was hardy any difference and that both courses will allow you to teach Psychology and other Social Sciences once you qualify (e.g. Health and Social, PSHE, Citizenship). However, if you definately want to teach Psychology then I would suggest taking the Psychology PGCE and apply for the Social Sciences PGCE as a back-up; this is what I did.
I think (if anything) you may end up teaching all social sciences on the Social Sciences course and will have to learn the subject knowledge applicable to all Social Sciences (you will have seminars at uni which teach you how to apply your knowledge of the subjects at degree level to fit the KS4/5 curriculum). On the other hand, the Psychology course will be more biased towards Psychology.
Finally, I was told at my interview that I would be able to do my placements in South Yorkshire and travel in to uni one day a week to attend lectures etc. This really helped me as Manchester was the closest uni that offers a Psychology PGCE and being able to commute from home will save me some money now the bursary has been withdrawn.
Hope that helps and feel free to ask if you have any more questions