It will be stamped in line with the expiry date of your health insurance as long as previous extension still in date and a new insurance obtained and presented to the immigration officer.
Midweek days cheaper than weekends when booking a week yet go according to the lowest advertise price most likely being Tuesday and wednesday. I usually find agoda best value for money and certainly easiest to navigate app.
Do register with a GP immediately. Everybody has a right for primary care on presentation (in person, not on presentation of documents). Hospitals may however require further documents proving entitlement to care but by then you will be likely a resident in uk for 3 months and entitled to care (GP referrals are usually not scrutinised though). We complain about the NHS however when things go wrong privately, treatments abroad, not insurance covered or uninsurable it always fall on the NHS to pick up the pieces.
Non-OA requires a health insurance at aprox 35,000 pa with little value and benefits. It also requires a medical and security check. Non-O simpler, cheaper and more flexible. I was attracted by the way it gets extended when returning to Thailand (along the validity of the insurance policy) but at the end applied for non-O in home country (which also simpler and cheaper than applying in Thailand).