If the rental is advertised in the UK then yes, you can rely upon breach of contract and breach of the Consumer Rights Act 2015. You are also correct in that one-sided terms for cancellation are also unlawful in EU law.
You should send the rental company a letter before action to say that you believe your consumer rights have been infringed under the above Act and that you demand repayment of the further payment within 14 days, failing which you will lodge a small claim at the county court business centre (which then gets sent to the rental company address).
The attached letter is generic and needs to be tailored (and quote the above breach).
You can use the European Small Claims procedure in order to claim.
Assuming they do not respond or they do and refuse to pay, you need to fill in the attached Form A and attach any evidence you wish to rely upon to :
County Court Business Centre, PO Box 527, Salford, M5 0BY
The form asks for your bank details and the 25 euro issue fee will be deducted but added to your claim sum.
The court will then serve the form on the rental company who then has 30 days to respond. The court will consider the response and give judgment on the claim (or if further details are required the court will write to either party). They may hold an oral hearing (by telephone if the court has the facilities - which the majority of UK courts have) but with the facts of this case it is unlikely as it should be capable of being dealt with on paper. If you win you get a certificate against the hotel which means they must pay your claim.
With the letter before action I have uploaded a template for you - you can email it to the rental company and include your bank details so they can pay you (ensure you give them the IBAN and BIC code, name of your account and name of the bank).
I hope this helps - if you can please click 5 stars at the top of your screen, I can answer follow up questions at no extra charge and Just Answer will credit me for helping you today.