Re: Your Return To The UK: You'll probably be aware of anyway as you are from UK but:
1. Airport testing on arrival is uncertain (depending on if/when it might start and what it might be). Many think it will be a double test, 1st on arrival, 2nd a week later. So likely you'll have to quarantine for a time and with big fines you might want to check you have food/supplies stoks ready before you depart UK (or means to get them whilst in quarantine).
2. Given the dramatically worsening situation in the UK, rules may easily hange whilst you are away and might be you can't spend Christmas with anybody anyway. Probably depends a lot on where you live and your personal circumstances but the question as to whether or not your visiting girlfriend is "a member of your household" might be very significant (plus is any of your neighbours are like UK's Home Secretary Ms Priti Patel who says people should call the Police about neighbours breaking rules!).
3. Your Travel Insurance: Double check and make sure your travel insurance covers you. Being from the UK you'll likely have a UK travel insurance policy and most are void if you travel to a country where the UK FCO (or whatever they call themselves these days) has a "...advises British nationals against all but essential international travel". There are a few specialist companies providing policies for countries where FCO "...advises British nationals against all but essential international travel" but they have age constraints and are more expensive.
Ian
[ Edit: Edited on 09-Oct-2020, 22:51 GMT by Psamathe ]