Hi,
that is such a good question - I can feel your dilemma.
First question: Why was your Manager able to offer your services? Are you not fully booked?
Second question: Have all other avenues been exhausted?
And what do you mean by your efforts were "hugely underappreciated"?

Although I don't know that much about working for an NHS, I think, there are many factors that play a role here.
a) Your actual contract and job description. What does it say? Does it have a clause which allows your manager to deploy you to other tasks within the NHS as required/ indicated?
b) You're right - washing is not within our scope of practice. It is a general task that everyone has skills for - so falls into your general rather than professional competences. Dressing however, is. So... 50/50.
c) What about the people who have it in their scope of practice, like OTs?
d) If it happened before - has anyone notified the union rep then? What did they say?
e) You're feelings - the work wasn't appreciated. Would you do it again if it was appreciated? Why. Your feelings about doing the job are irrelevant to an employer in emergency situations. Tough, but that's what it is. I would not bring it up in a discussion but stay with the legal and professional aspects.

I wouldn't like to do it either. Just saying.
Cheers,
Fyzzio