Oh No no no - sorry. What you're saying doesn't ring true.
Your initial question was 'should the carers be paying tax on any of the perks they get'.
That you now say the lad has his own (considerable, by the sound of it) funding, a court appointed guardian, an advocate, and that the whole thing is done through an agency, which presumably has been vetted by the local authorities, and are asking questions about how the money is spent, is totally at odds to your original question.
The question you now seem to be posing is whether this lad's money is being spent wisely. Whole different set of criteria.
The care workers would have to provide an action plan for the agency, and this would have to be agreed with any advocate or guardian prior to booking any activities - especially if it involved paying large sums - so that the money for this could be released in advance. The agency would not pay out for courses, then bill people, as they couldn't afford to take the losses if several people decided not to pay up for any reason.
As I understand it the care workers are not directly employed by the advocate or guardian, so any money they get would not be coming from this lad's estate. It is a personal matter between them and their employers (in this case the agency). If you are suggesting that the agency is invoicing the advocate for personal expenses on behalf of the carers then I would suggest that :-
A - A proper advocate is appointed, as only an idiot would agree to paying the expenses of someone not directly employed by them
B - someone contact the local authorities and ask them exactly what the agency is entitled to invoice for.
C - or change the agency providing the care.