Mnistooaddictive - the high workloads come from the sheer numbers of students you have. In my old schools, I regularly taught one class per hour per week. Therefore, it was not uncommon to teach 500 students+ (as most of your classes are KS3). Homework used to be set once a fortnight, but that's still 250+ books to be marked every week. Then, you because you teach so many students you have to go to every parent's evening. Chances are, only a handful of students / parents will actually want to see you - but you probably will be expected to be at almost all parents evening. At one school I taught in there were two parents evening per year group, so most teachers only went to one or the other - the RS teachers were the only ones who used to be there every time! Finally, it was no uncommon to teach 4/5 sets per year - so at report time, I could be expected to write 150 reports for just one year group etc...
Also, without a nationally agreed syllabus, there are very few good text books that will meet the demands of your local syllabus. Of course, there are some books that you can use, but most RS teachers I know tend to make their own resources.
However, I taught at high achieving, academic schools, so in those type of schools the work load / expectations can be higher (For example, in one school every report was expect to be handwritten, copying and pasting was not allowed, and had to be a 250 word individual statement about each child - fine when you're only doing it for one student - but when you have to do that for 150 students, it does become hard work!).
Not trying to put you off lankylto - but I think part time is probably a very sensible option. I found the marking / planning aspect very hard in my PGCE and I had no distractions. But, I will emphasise that it is totally worth it, and 13 years on, I still have a job I love and adore and would not change my subject for the world - it is the best .