Another vote for planning lots of things you might do, rather than expecting to feel sad about it even this far in advance.
In this situation, I make it a point to find out just how many things I could do. Staying in PJs all day watching TV and eating luxury food is much more fun if you are actively choosing to not do something else rather than listlessly not having anything else to do.
As well as the usual volunteering opportunities , I'd be tempted to consider something like the mid night service at King's College, Cambridge. Find out how to get tickets, when I last looked it was a case of dressing very very warmly and just queueing g for hours. Might not be your thing but crowds in those kind of situations do tend to be quite warm and friendly so can be a good all round experience.
Churches in London can also be very welcoming, especially the minority group ones such as the Scandinavian ones. As a family we lived abroad for many years and would always scoop up and visitors to come home for what would turn out to be quite a chaotic drinks and nibbles kind of event.
There are B loads of these kind of things happening up and down the country. The trick is to find the ones that appeal to you. Meetup.vom might be a rich source. They may be events closer to home which would be very glad of an extra pair of hands to help with their arrangements.
A morning walk flowed by an indulgent lunch and an afternoon of entertainment from the TV Or streaming services might not be so bad after all.
All the best.
Have fun in finding the really wacky options and, if that's what you