I think different areas do face different issues, though. Ex-mining towns in Wales may be more geographically inaccessible than some in other parts of the UK, for example. Some ex-industrial communities can turn to tourism, some can't. Some towns are commutable to big cities and some aren't. In Wales, you do also perhaps have the feeling of national identity which may make people more reluctant to leave to England.
I know it's not as simple as "just leave"- and obviously it is sad to see communities split up, but in general, I think young, ambitious people are a lot more mobile than they used to be. It is not uncommon for people I know from school/uni to move across the country to one city, and then maybe move to another area. If they're in a long term relationship, they want to be based near a city (not necessarily London) so that both partners can have opportunities to progress their career.
I really can't see how you can create that sort of set up in an area with lots of small/dispersed towns which don't necessarily have the best transport links between them. And tbf, if you do that for Wales, you probably do need to do it for areas of SW England and some areas in the north as well.
The problem with trying to attract businesses is that IMO you need all the infrastructure first, and if they just don't move, it's a lot of wasted investment.
However, I do agree that sometimes it doesn't actually take much to tip a community from being relatively economically depressed into being relatively prosperous. If you could attract in some big employers, and develop tourism perhaps, then more people would have more money- more money in the local area feeds into other local businesses, more shops and restaurants may spring up, large supermarkets may expand, small sole traders get more business. Over time, this feeds into people maybe being more likely to buy a house, with all the associated economic activity that brings, and therefore more house building etc too which can generate a lot of employment. This then brings a virtuous circle with an area appearing more economically attractive and more businesses setting up/moving to the area. And I do think this has happened in some northern cities etc. But I also think this is much easier to do in a city than in a cluster of small towns.