Whilst I was a non-resident I was entitled to reclaim VAT on purchases
made in the United Kingdom on goods I took out of the UK. The process
was that you had to take the goods and a form you got from the shop
where you bought the goods to a counter in the departures lounge run
by what is now HMRC - show them the goods and receipt documentation,
get the form stamped. Put it in the prepaid envelope provided by the
store and post in the post office box just outside so that the store
would reimburse your credit card etc. The system was fraught with
difficulties as if you had checked the goods in as part of your hold
baggage they would not stamp the form - if you paid cash for the goods
the store would send you a cheque - not easily cashed abroad. IIRC
they did not sell stamps at the airport shops in Terminals 4 and 3 at
Heathrow. Does it still work that way?