Correct I will try again in simple language
UK border agencies civil, frontier, intelligence and health have advance ACCESS to the passenger manifest of EVERY schedule arrival both via sea and air into the UK.
Entering the UK via the frontier post /passport control/Police stations you are obligated to comply with relevant English/Scottish/Welsh/NI/Bailiwick or Ellen Vanin public health orders. (Public Health is a devolved issue)
If you are transiting and haven't crossed the frontier your name and flight details remain known and shared with other nations, however you will be subject to the C19 regulations at your final destination.