Just four weeks ago, I had a flight from RAK via MAD to LHR where I arrived around 6 pm and my flight to Miami left the next morning. T5 closes for the night and you have to go through the automatic entry portal which was easy. Went outside, booked a Marriott Hotel and took a local bus there, your credit card would do for the fare and then came back to check into my MIA flight. The PCR test done at RAK was enough to transit MAD and also in LHR as well as MIAMI. The LHR security check I have always had back luck and t remember getting the same lady agent who checked me two years ago and she opened everything as i was travelling with just hand luggage. The new rules in the USA requires one day before test, one month ago it was 2 days so your test taken in Paris the day you leave should be fine for LHR as well as USA. I have a flight in reverse on 22/12 but Morocco is closed so I am not sure what would happen but I plan to do the same in LHR and MAD but only this time I may be stuck in MAD. Good Luck to you. I understand the confusion as no one seems to be able to help you with the up to date information.