This post is from someone who has had to spend hours sorting out platform movements, so that a train running on a Saturday service on a public holiday doesn't block in others that are running on a Sunday service!
Irrespective of when the official bank holidays occur (as, of course, they're moved when Christmas Day and/or Boxing Day occur at the weekend), the rule is that it's (the real) Christmas Day when there are no trains at all and (the real) Boxing Day when there are no trains except for a few airport services. At other times over the Christmas period there are modified services.
So this year train services will end early on Saturday 24 December. There will be no train services on Sunday 25 December. Monday 26 December is a public holiday and there will only be a few airport services running. Tuesday 27 December is a public holiday (replacing the Christmas Day one), with modified services running. Wednesday 28 December is a 'normal' day, except that many rail operators run modified services (particularly on key commuter routes) between Christmas and the New Year, as there's less demand for services.
The definition of 'modified service' varies between routes. (Around here we get a Saturday service on some routes on public holidays but a Sunday service on others. Either way, some early morning services may be withdrawn). My guess is, though, that you're most likely to encounter a Saturday service on Wed 28 December (possibly with no trains very early in the day).
As has been mentioned though, there is usually lots of engineering work going on over the Christmas period, so you could end up on a bus instead of a train.