It's worth noting that some Tocs prefer a Sundays outside the working week arrangement (voluntary). Reason being, it can be cheaper for them. Rosters based on 6 days as the working week mean you can work the entire rosters on principals of groups of 3.
I.e. 1 on earlies, one on lates and one on a rest day at any and all times. Plus a few people as relief. It works across guards, conductors, drivers, station booking offices, dispatch.
So you can staff a station 18 hours a day 6 days a week with 3 staff plus a floating area relief. Now put Sundays in, you may end up needing another 2 permant staff. That's 2 more lots of expense for the TOC, national insurance implications, pensions, benefits. A good many Tocs would rather run with less permanent staffing costs and cover Sundays as voluntary. Especially over stations which, if they go unstaffed for a few Sundays doesn't really bother the TOC all that much, after all, they're not paying the wage if they're not there.
The downside is of course you get periods of cancelled services on Sundays where train crew choose to take their days off. Public perception is obviously quite negative about this at times but it's the necessary downside of running with less staff and having Sundays as voluntary overtime. It may work out more economically sensible for tocs to run this way, paying the fines where trains are cancelled but not having a larger long term staffing bill. Perhaps some of them have decided this way is cheaper than running all services but needing a shed more permanent staff.
Also the railway does tend to be rather dead on a Sunday morning outside of London and the other main capitals. It only really kicks off at 10 or 11am.