(2-part post):
Just to 'tidy up' the previous answers:
There's a difference between 'duty-free' and 'duty-paid' goods. There used to be a 'duty-free' allowance which permitted you to bring in a very limited number of goods, from any overseas country, without paying duty or VAT. This allowance still exists for travellers arriving from countries outside of the European Union but it's long been abolished for travellers arriving from within the EU.
When you enter the UK from another EU country, you're now allowed to bring in an ***unlimited quantity of goods without paying duty or VAT. This is because you will have already paid tax and duty (at the prevailing local rate) when you purchased the items. This will apply wherever you've bought the goods abroad,either in the town or in the airport. (The only word of caution here is that, if you do see an outlet offering 'duty-free' goods at a foreign airport within the EU, you shouldn't purchase from there. A well-regulated airport will make sure that this facility is only available to passengers travelling to/from places outside of the EU but, if you did find yourself able to purchase items from one of these outlets, you'd have to pay duty and VAT on arrival in the UK).