Is it possible to assist on a voluntary basis i.e. no contract, no salary, no commitment, just help out now and again without endangering a retirement visa?
, no idea Transfer Wise may be able to provide an answer. It may simply be that a reason must be supplied to the UK and they have just added it, as a logical reason, to the list.
I believe that one of these options (any one of them) must be selected to comply to UK law and has nothing to do the Thai athorities or transfer company's requirements.