In my opinion, Dubai Duty Free is way better, and in many cases cheaper than the LHR. Check out http://www.dubaidutyfree.com/index.asp. There selections are way better than LHR too.

You can use credit cards or UK pounds in DDF, so you dont have to worry about taking AED with you either.

If you do however, want to buy in the UK you can take liquids though in transit without problem if it is in the specific bag.