Come with a single entry tourist visa from the Thai embassy in London. This will give you 60 days when you arrive and you can apply for a 30 day extension one time. That's 90 days.
OR apply for a Multiple-Entry tourist visa from the Thai embassy before you come. This is valid for 6 months from the time it is issued, and will give you 60 days every time you enter Thailand within those 6 months. That means you can do 60+30, do a quick border bounce or take a short trip to a nearby country for fun and return, 60+30, and then right before the visa expires around the 6 month mark, do a final border bounce for another 60+30. I can't imagine 9 months isn't enough time for you to resolve what you are waiting for.
Once the money is settled you can then convert to the non-O visa within Thailand once you have 800,000 baht in a Thai bank account. Getting the other spouse on a trailing visa for that is a little bit more complicated unfortunately as it will involve you having to leave Thailand again and apply for the non-O trailing spouse visa from a nearby consulate. Or you can each get 800,000 in a Thai bank account and then you can both apply for the non-O visa directly from within the country.
I'm not sure of your finances but you can also look into one of the LTR visas and see if you will qualify for that. It's a 10 year visa and if you qualify it will make everything easy for you.
**********************
Your airline is the only one who will care if you have a one way ticket, so ask them their policy. If they want to see a flight out of Thailand then google "onward travel ticket" and you can rent a plane ticket to use when you checkin for your flight to Thailand.
You'd probably need to use this as well if you wanted to apply for the Multiple Entry Tourist Visa since for that application you need to show a flight to Thailand, a flight out of Thailand within 60 days, and another flight back to Thailand.