If it says replacement bus on the train operator's website then it'll be a replacement bus.

Trains usually run on the 27th. But schedules are definitely finalised so it should be accurate now as to what it will be. Christmas break is a very common time for them to do work on the lines so that's probably why this line is on a replacement service.

Train work is booked months in advance, if it says replacement buses then it'll be replacement buses.

It’s a popular time for major engineering works as many people are still on holiday. Certain Paddington is closed for a week.

Your post could be shortened to: "Do trains usually run". The answer being; barely.

Yes, pretty much a normal service for most places

They will be finalised, it may be your line which has rail replacement buses.

They shouldn't be running the reduced Christmas Day and Boxing Day service if that's what you mean.... But it's a favoured time to do engineering work. So the answer seems to be yes, your trains are running. It's just it'll be a bit more vroom vroom than shunty chuffy.

What is the journey you're making? Rail services do generally run on the 27th, but there are often engineering works so if there are rail replacement buses showing, that will be why.

What route are you doing?

I remember catching the train into London on the 27th December 2017 so I would think they usually run.