All they have to do, if ringfencing as proposed is deemed to be not legal, is for all 13 Clubs to resign from the PRL and set-up a new, ringfenced, league.
The RFU might, initially, decide to not recognise it, but with all the England players and the BT and CVC money they'll soon cave-in, otherwise it could lead to a pan-European club league answering only to itself.