You can pay to be tested, I had one before a flight. It was £200, or £250 if you needed the results within 48 hours. The results get emailed to you.
If you’re having a test without symptoms, you should pay for it; really. I’d hoped that the price would drop but it seems fairly stable, although I haven’t researched specifically for Christmas as the in-laws are over 65 and it doesn’t seem worth mixing. Our local hospitals are approaching capacity with over 60s, and I don’t want to risk them ending up there.
If you go for an nhs test, you’d have to lie about having symptoms, and then the results will be back whenever they’re back - I’ve had local friends have them back within 24 hours and take almost a week recently (both did have symptoms).