Sue (POC user) said:
the room they have given me is not a downgrade, it is unusable
Sue, that sentence is the one to use with them, more or less word for word, because it is the whole argument. This is not a room change, it is a failure to provide the thing you booked and paid for, and the difference between those two framings is what decides how the conversation goes.
What to do in the moment, in order. Ask for the duty manager by that title. State plainly that the room provided is unusable for you, not merely different, and say why in one specific sentence: the bathroom has a bath and no level access shower. Ask for one of two outcomes, an adapted room here tonight, or an adapted room at another hotel with the transfer paid, and say you are content for them to choose which. Do not accept the standard room even for one night, because occupying it is treated as accepting it. If they cannot house you tonight, ask them to cancel and refund in full so you are free to book elsewhere, and get that in writing before you leave the desk. Keep it calm and factual. The staff member in front of you almost certainly did not cause this and will help you more if she is not being shouted at.
On what you are entitled to, the honest answer is that it depends on where you are and how you booked. Consumer and equality law differs by country, and a booking made through a third party site often means the site, not the hotel, holds the contract, which slows everything down at exactly the wrong moment. What is consistent everywhere is that a documented complaint made at the time is worth a great deal and a remembered one made later is worth almost nothing, so Derek is exactly right about writing it down tonight.
Then the prevention, because this is the bit I have changed in my own booking habits after it happened to me in Lisbon. I now ask for the room to be flagged as an access requirement rather than a preference, in writing, and I reconfirm 48 hours before arrival, which is late enough that they cannot have quietly resold it and early enough that they can still fix it. I also ask one blunt question at the time of booking: is the adapted room ever released to other guests, and under what circumstances. The answers vary and the hesitation tells you plenty. That question and the rest of the list are in the questions to ask before booking, and what a genuinely adapted room should contain, so you can describe precisely what is missing, is in accessible accommodation.
What I cannot tell you is what you can manage for one night, because a bath with a shower over it is impossible for some people and awkward but survivable for others, and only you and your own therapist know which you are. Please do not let tiredness push you into a transfer you would not attempt at home.