Watching with interest as my aircraft is based at Hawarden so is one mile inside Wales. I'm sitting at home in Cheshire with GA activity buzzing in all directions but I can't fly until the Welsh Office revue the current rules on the 28th. I've done my health and engineering test flights and it's another two weeks until my next permitted trip. If they don't change the rules this week I'll remove it to England and have to pay for two hangarage places.
As for overflying Wales from England then the same principle as driving applies. You didn't need to go there, you've no reason to be there and if you wind up sitting in a field there you're on your own for an explanation. Your insurance company might also take a dim view if they were so minded to get out of paying. For now it's worth staying on the right side of the border.