Well if you have been in a red country, even just as a transit passenger, within 10 days of arriving in to England then you shall need a negative covid test before departure and upon arrival get banged-up in a Boris B&B for 11 nights at a cost of GBP1,750
An amber country is similar, a covid test before departure but upon arrival in England you may self-quarantine for 10 days at home or in a rented accommodation that you may have booked but upon arrival in England you must travel directly to that accommodation and not take any deviation en-route, once in that accommodation you are not permitted to leave it until the 11th day.
Scotland and N. Ireland rules may vary and last I checked there were no such flight arrivals permitted in to Wales.
As n newbie member I'm not permitted to post links but google for "How to quarantine when you arrive in England" and look for a gov dot uk site amongst the results