I am not sure how militant Englands fauna and flora police are but I have seen the extremes that Australias Fisheries department will go to, to erradicate pest snails from commercial fish tanks. So whether or not snails that are currently in a country can still be traded, I wouldn't be willing to test the authorities resolve in not continuing to have them traded.
In Australia we techniqually dont legally have cherry shrimp, bee shrimp and crystal shrimp, but many people do have these species in their tanks. AQIS knows this and for now these shrimp currently in the country (even though they where smuggled in at some point) are allowed to be kept bred and traded, however some Australian forums have put bans on sales and trading of these intorduced species of shrimp to avoid later legal issues should AQIS ever decide that all non native shrimp need to be erradicated.