You could try (if you didnt already) the "coronavirus immigration team" (CIT), see below (from thread
If you intend to leave the UK but are not able to do so by 31 August 2020, you may request additional time to stay, also known as ‘exceptional indemnity’, by contacting the coronavirus immigration team (CIT).
The indemnity does not grant you leave but will act as a short-term protection against any adverse action or consequences after your leave has expired.
The Coronavirus Immigration Team will provide you with further advice on what you need to do to request an indemnity. This will include providing details of the reason why you are unable to leave the UK and supporting evidence, for example, a confirmed flight ticket with a date after 31 August or confirmation of a positive coronavirus test result.