So can anybody suggest some plausible reason why taxpayers should be expected to pay these expenses to people whom they no longer employ, and who are already receiving very sizeable pensions?

Is there some view that they are still acting as representatives of the British public when they are invited on public speaking engagements? I suppose it might be argued that if they are not reimbursed for things like that to cover expenses - but any expenses or payments paid to the exPMs for such speaking engagements ought to be taken into account in estimating the expenses payable by us should be taken into account, and that doesn't appear to be the case.

What people think of Thatcher as an individual shouldn't be seen as relevant in this context.