Some GEM universities do not require Chemistry A level, for example, Swansea, Warwick, SGUL, Nottingham and Newcastle, and probably a few others. So the choice is either do the A level at night school (although it can be hard to find courses, since labs cannot be done by distance learning) or apply only to schools that do not require it.
FYI I asked Cambridge if they would consider me without Chem A level since I have studied there before, but they said they couldn't make an exception and I would have to take the night school option if I wanted to apply there.