Britain had a dual tax agreement with Thailand, so you won't be taxed twice on your pension. I am also on a UK pension and I enquired about what to do, if anything, and that still appears to be a grey area.
Unfortunately there are a few paid up members of the "you didn't want to do that" club who just have to be critical. It must be great to be perfect all the time and never make a mistake!
I get the state pension from the UK and have twice had to do this over the years.
First time I got my doctor to do it but she charged for it. The second time I got the condo manager to sign and put his building stamp on it. Both were accepted without a problem
In our case, it isn't sent out. The meter reader comes to the house, reads the meter, puts the reading into his computer and that generates our bill that he leaves in the mail box. You can't pay it straight away because he hasn't downloaded the day's data to the main company database. Once he's done that I scan the barcode and pay
Safety boxes come with a master reset code set by the maker, something like 0 0 0 0 or 1 2 3 4. Hotel owners rarely change this master code and thieves know that.