That is very much the point - if you look at post 2 https://www.railforums.co.uk/thread...placing-off-peak-returns.216411/#post-5092775 there is talk of a 'Bristol area PAYG scheme' and a 'Devon and Cornwall PAYG scheme'.I guess you could have islands of contactless PAYG around each city, but the problem I see with that is the commuter belts of british cities overlap.
There are overlapping commuter areas in this country but you just have to draw a boundary point - notably Ashchurch is in a potential Bristol scheme and Severn Tunnel Junction isn't despite their relative proximity to Bristol.