Plenty of people out there, just not at the price industry wants to pay.
I'd hazard a guess that the reason some companies are having trouble recruiting is that they go to their favourite outsource partner such as IBM, HPE or Accenture and find that they don't have any offshore staff that can write for mainframes.
There are plenty of FORTAN and COBOL programmers out there with plenty of years of work left in them and we also know how to write lots of new whizzy stuff like c++, python , java and whatever other languages are flavour of the month. I just tend to keep quiet about the older stuff because employers don't seem to be recruiting for the older languages.
And I expect that the outsource partners have plenty of UK based people that can code for mainframes but keep quiet about it because they have had "to move up the value chain" and take PM, scrum master and other similar salary preserving roles as coding jobs have been shipped offshore.