The UK Government and devolved Governments do not accept negative covid tests on arrival at this time. You fill in the passenger locator form and are allowed to transit, directly, from entry at LHR to your place of self-isolation, presumably near Aberdeen. So you are allowed to take a transfer flight to ABZ and drive from their to your accommodation. You are allowed to stay overnight at a secure location (like a hotel you have told you are self isolating) if needed to get to your place of self-isolation, i.e. if your flight to ABZ was the next day or you can't drive from ABZ to your home/accommodation on the same day. In that case you fill out both locations on the form. You should go directly to your accommodation ad not use any public spaces and not go shopping on the way.
https://www.gov.scot/publications/co...ks-at-borders/
The 'Test to Release' scheme that begins on 15 December 2020 does not apply to Scotland, only England. So you need to self-isolate for the full 14 days after arrival, or until your direct travel back to an airport from which you leave the UK.