I have applied for my COE and had pre approval from the Thai Embassy in London.

 

COVID insurance done, trying to book flights, had intended flying out on Oct 14th, the flight I provisionally chosen has been cancelled, but available next day. If I book for the next day 15th does this effect my final approval for COE and insurance (which will expire several weeks after my return to the UK)

This from my COE:-

 

"Note to airlines: Flight changes (due to flight delay or cancellation ONLY) are permitted within 72 hours from the original flight time without the need to issue a new Certificate of Entry, provided that all other required documents are still valid."

 

If you book the same flight with the same airline but just a day later then I don't think you will have a problem.

 

Can't see any problem with the insurance as you seem to be covered for longer than your stay.