Yes it is worth it. I'm currently on a gap year working in a GP surgery (admin side). I can say for sure that I've learnt loads abut the NHS and how care is delivered.
I've helped with appraisals, extensively used EMIS PCS, gone to practice managers and GP partner meetings discussing how PCTs should fund for certain services around the local area, helped with QOF, done DES report for patient participation, summarised medical records, updated NHS choices, actioned FP69 forms, ... List goes on! But basically my knowledge of primary care organisation is if I may say so very detailed compared to before I started 7 months ago.
Although you wouldn't do all of these as you would only be doing work experience and not employed I can assure you seeing how NHS primary care is run is so useful. Even if it is just filing!