Might be worth taking a look at the sale of goods act
Sale of Goods Act Fact Sheet - BIS
Q7. Do I have to produce a receipt to claim my rights?
No. In fact the trader doesn't have to give you a receipt in the first place so it would be unfair to say that you had to produce one. However, it might not be unreasonable for the shop to want some proof of purchase, so look to see if you have a cheque stub, bank statement, credit card slip etc., and this should be sufficient.
Consumer Direct - Sale of Goods Act 1979