Based on my limited understanding of the law.
1: If you have the means to pay and it is offered, you have not broken any criminal laws.
2: There are civil laws that would cover it, but these would be very hard to take action on if the following (some or all, I cannot say) criteria has been met.
A: You have legal tender on you and have offered it as payment.
B: You were not implicitly advised that they only took specific types of payments before using their services.
C: You offer them your contact details, so you can return at a later date to pay.
If they call the police, they should be informed it's a civil matter not a criminal one and in no way should you be detained or harassed in anyway, which could be construed as criminal.
if anyone has a far better understanding of UK laws... please feel free to correct my misinterpretation of things.