Statins are usually allowed under mil rules so I would expect CAA to be OK as well. They may, however, require you to have a period off flying P1 until it has been established that you haven't got severe side effects. A bit of stiffness is usually bearable but very painful muscles is rather different. Check with your AME or your licence issuing authority if there is a specific list of approved statins. It may not be the same as the list your GP has!