As a Thai tax resident any UK dividend income is paid out tax free. Find yourself a decent brokerage, I use low fee Degiro, and pick funds or shares based on your risk tolerance that have historically paid decent dividends. Aim to generate stable passive dividend income to supplement your pension. Avoid anything that has a platform or fund management fees.
You need neither. To obtain a Thai TIN you need your passport and proof of address. For proof of address a residency certificate, a yellow book or a rental agreement will suffice.