I have a flight from Manchester to Heathrow at 10:40 connecting to an inward flight to Las Vegas at 16:40.

Am I going to need to take the COVID test prior to checking in at Manchester or prior to the Heathrow flight.

Also with the USA rules stating that the test has to be taken no greater than 24 hours before your flight then would this mean prior to the Heathrow to Las Vegas leg.

if so then I will need to sort test between 16:40 the night before and about 08:40 next day when I check in at Manchester.
not a lot of time.