Well, everything on the iPhone Upgrade Programme site for the UK indicates it's in-store only:
https://www.apple.com/uk/shop/iphone/iphone-upgrade-program
To be sure though, I strongly suggest is giving Apple a call to see if they've made other allowances in the UK due to Covid-19.
You can call them directly at their UK number listed here:
https://support.apple.com/en-us/HT201232
Since it's a rapidly evolving situation though I would be surprised if any arrangements have been made yet, but that's just my opinion.
You can use the https://getsupport.apple.com website to arrange for a scheduled call-back and save the toll charges if you don't have a land line.