Upnotover

It has nothing to do with the embassy, applications are received at VFS offices in Bangkok and Chiang Mai. Look here for all the information needed; Overseas British passport applications – Apply for a passport – GOV.UK It is highly unlikely you will get your new passport by October, currently they estimate 11 weeks. You may need a trip to the UK to get sorted, or perhaps leave and re-enter on your Irish passport and do the same when the new British one shows up.

Yes, yes and yes. From the UK embassy website; Single Entry Visa will be valid for 3 months from the date of issue. You must enter Thailand within the validity of visa. However, the permitted length of stay is up to 90 days from the date of your arrival in Thailand. Please carefully note the immigration stamp in your passport to make sure that you do not overstay your visa.