You're question seems a little confused- do you want temporary work experience as a signalman or do you want to know what qualifications/part time work you need to undertake?
If you want temporary work experience on the 'real' railway - it's not going to happen, sadly. You may find it useful contacting some heritage railways to see if you can go there as they may have working signal boxes. They tend to switch the 'automatic' ones to 'manual' in busy Summer times.
Study:
English Language, Maths, Biol, Chem & Physics [Sciences] are always the essential building blocks.
Perhaps think about Design Technology- Electronics.
Geography (You will have to deal with co-ordinates/maps)
Physical Education (PE) Always good to be fit- and know how to keep fit. Useful for medical(s).
Computing/IT
and then if you can do any more subjects try to do things you like doing, not particularly linked to being a signaller.
You may find it useful emailing Network Rail and asking for a 'Job Specification' for a signaller. It will give you very detailed and precise information, probably quite technical in parts.