Remember the duty free allowance applies to goods purchased or bought into the UK from abroad, i.e. outside the EU. So when you buy your Rolex, Cartier etc at LHR this should not form a part of your customs allowance since it is not something purchased or bought into the UK from abroad. On the other hand if you buy a Rolex from outside the EU and bring it back to the UK then that should be counted as part of the allowance, will take you outside it and should be declared to Customs in the UK.