To be safe you should take the lid off: very easy and instructions are somewhere around the forum if you search. I see you are a gold member so take a look at the service manual!
Keep in mind the paint stripper will melt any plastic away upon contact so be very careful on the edges of the metal, work bit by bit.
You can use any laquer stripper, nitromors will be fine as it is probably available in the uk. If not there is probably something with similar ingredients, check at your DIY store.
No guarantees on what you will find underneath the paint though: someone once told me the panels that were turned down in the factory for beosystem 4500 were sprayed grey and used on the cheaper 3500.
Good luck and remember to post a picture of the result!