Retirement visa done in Thailand is the simplest. Get a 60 day tourist visa in your home country which can be extended for 30 days, the 90 days will give you plenty of time to sort out the retirement visa.
In the UK, your latest will is the only one that is valid, all your old wills are null and void. In Thailand I do not know, seek legal advice in Thailand.
The most important thing is that she will return to Thailand - has a good reason to return, good stable job, a house land etc. you will need evidence of this.