Dave
Honorary Master
You can order a Giff-Gaff sim for free to be sent anywhere in the world and then activate it when you are in the UK. I selected £10 "goodie bag" as they call it. You only get charged once you actually activate the sim. Ordered it today, will update when it actually arrives at my house.
Giffgaff’s recent changes to allowances (goody bags) makes them more competitive but I’d suggest a Vodafone or Three sim if data usage is important, Three have a PAYG brand called Smarty and Vodafone one called Voxi with decent allowances, their sub-brands offer better value than the direct PAYG offerings.
https://www.voxi.co.uk/plans
https://smarty.co.uk/