A level music and music theory grade exams are both very different because they assess different skills in different proportions. I personally found music theory grade exams enjoyable and manageable and got merit in lcm popular music theory grade 8 but I got a D at AS level music so it's different strokes for different folks and hard to know what to recommend really.
I personally believe that A levels in the arts are too rigid in their syllabuses and in how they're assessed and think btec and grades are the way forward but maybe that's just me.
I would recommend asking your music teacher at school what their thoughts are on you doing A level music but I think that it can be a lot of difficult work to get the higher grades at A level if it's not a subject you want to pursue (especially when there are other ways to pursue it). When it comes down to getting those all important ucas points music A level was a bit like flogging a dead horse for me but everyone is different.