Get the 90 day Non O based on retirement from your home country embassy instead of the 60 day tourist visa. Then, once in country, you only have to worry about applying for the 12 month extension of stay.
yes the UK state pension is taxable on remittance to Thailand. As it is below the standard UK tax threshold you do not have any tax credit against a Thai tax liability meaning once you remit above Thai tax thresholds it will be subject to Thai taxation.
UK pensions means what exactly? All pensions are taxed on remittance to Thailand unless specifically exempted through a DTA. In the case of the UK only civil service pensions can be remitted tax free, all other types are taxed.