RXTVlog have posted a follow-up tweet with a photo of the table OFCOM made.
https://twitter.com/RXTVlog/status/1148500499261509632
In this context I think this will not be as disruptive to the VM EPG as I first thought. Seeing it like this I think it's easy to misinterpret, I don't think it is actually saying regional channels need better numbers on UK wide EPG everywhere, just in the regions they are aimed at. Which is of course more sensible.
BBC1 Wales HD is on Ch101 rather than 108 I believe, so is 108 empty in Wales? If so, surely VM could put S4C on 108 much like BBC Scotland channel is on 108 in Scotland and 162 elsewhere. The omission of BBC Scotland being mentioned on VM in this table I would interpret as OFCOM being happy with this type of arrangement.
Which means the only change OFCOM want that will be a problem on VM is BBC Alba. Making space for one channel rather than two is obviously much easier to do in the 1 and half years they have to implement it though.