In theory if everyone at your level gets the same % and it's not 'discretionary' (ie not dependant upon individual performance) then no, you don't get it (hoyland v asda I think)
If it's discretionary then yes, you should get it. Plenty of places pro-rata it although most lawyers advise not to pro-rate (??madrassy?? case IIRC - Italian city lady but can't recall exact spelling). I think you potentially have a case if they pro-rate, but it is a very grey area indeed and I don't think there has been any recent case law to properly clear up whether people should get full 12mth amount versus pro-rated sum.