John, there are no direct flights from Crete to UK in Winter. You have to go Athens or other mainland airports.
Most winter flights go into London Heathrow, not sure if now the odd flight into Scotland, but it looks like you are in Wales.
You have not stated if you have a preferrance to Chania or Heraklion.
It is up to you whether you will use the same airline carrier to UK from Crete or 2 separate ones.
There has been some threads here in the past about claiming back some of the tax at Athens, I have not done it.
If you do use the same airline carrier from Crete to UK, My Advice
at the check-in desk on Cretfe make sure your luggage is checked through to UK
will save you having to pick-up in Athens and re-check it in – that you would have to do with separate carriers.
I was once made mistake at Heraklion & although I realised 2 minutes later, once suitcases had been taken, I was told too late.
The Heraklion flight was late taking off & had a big rush to pick up suitcase, get it onto UK flight, then had to run with hand-luggage for flight, made it by skin of teeth.
Shame as do like to do some shopping at the shops in Athens airside, they have bigger selection than Heraklion in Winter, also saves carrying it for that part.
The times have travelled in winter the flights from Heraklion were usually half empty, but suspect Christmas would be different.
You may choose to have a big gap in the times of your flghts at Athens so then you can do some sight-seeing in Athens, say 8+hours.
Think I normally had 2 hours between flights, so no time to leave airport – that suited me.