Your profile suggests you are British. Do the passport renewal NOW. You have presumably done a 90 day report last month so you have plenty of time... do not wait, appointments at VfS are Covid affected so the 'normal' one month timeframe may well be longer.
spot on...but, for clarity, the 15 days includes the due date so it is really 14 calendar days, so (for example) 24th March reporting date means that you can do online from 10th to 24th and in person until 30th (24th being day 1 of 7) .
You will be stamped in at Swampy... can take an hour at busy times. October should be OK... but you have to get from international arrivals to domestic departures. Far too tight for comfort unless you are 'transiting ' through Bangkok, which I doubt.
... whilst this may infer Thailand, it is not specific.I can imagine that it may e possible to evidence a UK address and supply a covering note that says you are staying temporarily in Thailand. The passports have to be collected from VfS so the address is arguably irrelevant.