I have a few friends who work their as data entry clerks part-time. I got offered a job but the money wasn't as good as what I do now. They are usually looking for new young people who can type well (there is a training thing if you aren't good enough). The only other qualifications you must have are GCSE's (A-C) in English and Maths.
This may not be totally useful as you live in Wales and I have no idea if the postal service is the same for Wales as England