Has Anyone Applied to Extend their Thai Partners UK Visa ?

My Thai partners U.K. Visitor/Tourist Visa expires on the 4th of September, our 5 year old son is here as well but has a UK Passport. We have been in a relationship for 6 years and co habit in Thailand currently paying rent for a house as well, my Thai Visa has expired. 

My options are as follows apply, not sure if they would be accepted as she doesn’t have a job to return to,
First one send the 2 of them back but it’s not ideal to expect a 5 year to spend 14 days in a hotel room, I would need to travel to London by train, I don’t drive due to poor eyesight, cataracts 400 kilometres, stay in a hotel till all the details for the flight details were finalised and a COE issued, they would need a Fit to Fly Certificates which the Thai Embassy can arrange I believe.

The second option is to apply as a family but the additional costs of fares, pre flight tests and the cost of the 14 day quarantine would make this option prohibitive so not really an option.

The first option would also be costly with 2 airfares, travel to London and accommodation.

The third option is to apply to  U.K. Immigration to extend my partners Visa, currently no decision will made on extending Visas till after the 31st of July. This would be my preferred option as we have 2 vouchers with B.A, for flights from Newcastle via Heathrow to BKK. B.A. are scheduling flights in September but after speaking to them this is subject to change.

Has anyone applied to have there Visas extended, I have ‘googled’ but can’t find any info on extending her Visa but I can’t find any info, any advice or assistance appreciated.