Because he says he has “plenty of cash in the uk and thai banks”. Unfortunately since you highlight that there is no lump sum option in London, that’s not a consideration anymore.
Well, Chiangmai Immigration is just next to the Airport, so I would advise that you plan to drop by after you land if you arrive early enough in the day. Otherwise, you have until 1600 hrs of the next calendar day to go back there. TM30 section does not close for holidays of any kind, even Songkran.
If you could visit a neighboring country by land and return some time around the 75th day, that would save your family a second trip to Immigration and 1900 THB each just for the denial extension.
By 1600hrs of the next calendar day of your arrival. If you arrive in the morning or early afternoon, just head there directly with your passport, TM6 and TM30 acknowledgement slip to save you another trip back there (unless you live/work nearby, of course).