I suspect some leisure travel domestically will drop again soon enough, once the world opens up some more then I would not be surprised to see services to airports get really busy again.
Quite, I have plans to travel into Europe in January, and I'm really excited about that. Possibly not by air, as it will depend where I actually choose to go and what is available when my new passport arrives. From what I hear, France is mostly open to the UK with some minor restrictions and I suspect that is what I will choose. Poland appeals hugely, and that appears to be an option for January for UK residents.
It will be interesting to see what happens in 2022, but I'd not be surprised if there was a resurgence of peak time travel.
As for current leisure numbers, TfW don't have much to worry about off-peak on the Cardiff to Manchester route, that line is busy all day long. I've yet to board a train on that line which isn't busy, with few to no seats spare and full of covidiots, regardless of the day or time.