There are plenty of threads about this in the UK & Ireland specific FT forum.
If items are in through-checked luggage, reclaim form should be shown (WITH GOODS) to customs staff prior to checking in the baggage.
If carried in hand luggage, they should be shown at the final departure airport exiting from the VAT tax area of the EU.
Note, a reclaim form must have been signed and completed by the retailer (who must participate in the retail export scheme). You can't just randomly shop and expect to reclaim it.