It's is illegal to import and/or trade in wild caught american bullfrogs in the UK (this is well know) but it is legal to sell captive bred (destiction of a captive bred is an animal where both parents were captive raised or captive bred (captive raised is 0 or only 1 of the parents were captive bred/raised)so basically second generation from the wild) american bullfrogs in the UK but since its almost impossible to prove that the animal in question is captive bred its always going to be legally a bit iffy.
I did a bit of research too- seems they are causing a bit of trouble down in Kent. Pet bull frogs released are surviving and breeding and causing all sorts of trouble for native wildlife.