Prices are not lower in Wales or the West Country (or the North, or Scotland) because houses are isolated. They are lower because they are not near London, and the transport links are poor, employment is less plentiful and there is less by way of free entertainment.
People retiring to 'non-Southern' areas are not saving the economies of their new home towns - they push up house prices, particularly on bungalows and purpose-built flats for older people, making them less readily available to local older people, and then pay less to local councils than local families if they are single-person households and get a rebate on council tax.
Older people tend to buy fewer household items, or items such as children's clothes that need to be replaced frequently. They tend not to stay out late, so local nightlife venues don't benefit from them living there. They eat less than families, so such remaining grocers, butchers and so one as there are will not benefit from an influx of older people. Other shops, such as clothes shops aimed at younger people go out of business, so New Look and equivalent get replaced by Seasalt and similar outlets, and the town becomes less attractive to young families and more so to older people. Larger houses are sold to care home businesses, and the cycle continues.
It's not necessarily North versus South, but it is far more likely that an older person from the South will buy property outside of it than the other way round, leaving the South with wage-earning residents who pay full council tax and use all the local facilities.