Sorry to hear of the issue.
It sounds suspicious if you didn't sign anything.
As they need a signature from you to mean you are legally bound to pay anything to them.
There are reviews online from others to say there is an electronic signature - and using scare tactics to get you to pay.
If you click here: https://www.hotukdeals.com/discussions/hidenda-threatening-lettersemails-3857918 you will see what I mean.
If you did not sign anything, you are not liable for anything. So you can refuse payment and perhaps refer them to the above website and ask them not to harass you again.
If you are being harassed by them, there is an option and that would be to apply to the local county court for a civil injunction. This a court order requiring them to do something or refrain from pursuing a course of conduct.
However, before you do this, send them a cease and desist letter to warn them of your intentions unless they stop (and give them 14 days to stop their conduct to allow them to seek legal advice).
A template “cease and desist” letter is available at this site : https://www.lawdepot.co.uk
If you go to the search box in the top right corner of the site, it will bring up a list - you want the "general" letter. You can use your own letter if you wish - it just has to make it clear this is a cease and desist letter, that you demand they stop their conduct, and that if they do not within 14 days you will apply to the court for the injunction and seek your costs.
That letter may well be enough to stop the conduct - as an injunction is a serious legal remedy. The judge will want to see that you have sent the cease and desist letter given an injunction is a last resort.