Your £75.00 is made up of £50.00 for a speculative invoice - disguised as a fine, and a charge for paying for that made by the car hire company that they have paid on your behalf - and shouldn't have done.
When you hired the car you will have used a credit card and the money will have been taken from that card. Therefore should you choose to listen to the people telling you to ignore you will be £75.00 short!
Start with taking a look at the the terms and conditions of hire that you signed. This will have a clause in it about passing on any fines and penalties you are charged. This is neither, but simply a speculative invoice disguised as a fine for £50.00.
The way to deal with it therefore is to write to the car hire company and ask them for it back. Of course include the £25.00 admin cost.
Of course they will not dream of sending it and offer all sorts of excuses.
This person got his money back in exactly the same circumstances and I suggest you use their letter to SIXT as a template to do the same. (The final letter that produced a result that is!) Note he got his money back and there is no reason why you should not do the same. Show them that they were wrong to accept the speculative invoice as a fine (and wrong to pay it). What they should have done is send it to you and tell UKPC that they had done so.
They are NOT entitled to pass on your name and address, which they have done - against the provisions of the Data Protection Act.