The 1980 Act only requires proof of purchase if goods are not merchantable. The Act doesn't mention a receipt per se.
This proof in the past was always considered to be a receipt or a credit card statement, bank statement, cheque stub, or unique Retailer Label on a product.
Today, e-mails are currently accepted as proof.
Also as mentioned by Sue Ellen, I think it's a great idea to have the guarantee detailed in the e-mail together with details of the product purchased. Of course we know from The Act that guarantees are in addition to the rights afforded by the Act. So, it's all good news for the consumer - particularly by having these now available in electronic format.
Having e-mails and texts as proof of purchase is an excellent idea and will be particularly helpful for those who pay by cash and or those who otherwise cannot provide paper proof (for example a lost receipt).
For those who prefer a receipt provided at the till - they should definitely ask for one if they are not currently being offered one.
Marion