AJM - if I'm being totally honest I don't actually know if this is guidance or law. I do know that I am allowed to, and do, cross the border multiple times a week to go food shopping, go to my local DIY store and local takeaway.
Interestingly, on the topic of Welsh/English restrictions, from April 12th a person living in Wales will not be allowed to go to their local wall/gym which won't be open until early May at the earliest. However they will be allowed to drive to say London and go indoor climbing there.