The SBS(R) and SAS(V) Selection and training will, by necessity, be less intensive than the Regular units. If for no other reason than it's is spread over a much longer period.
However, UKSF(R) will train to get as close to Regular standards as possible, so they can work alongside Regular SF on Operations.
The point of JSFG(R) would be to increase the amount of trained personnel available for Deployment in support of Regular forces. An ad hoc grouping of 4 Para and RMR would not work. That's why the Regular Para's, Marines and Rockapes who make up JSFG have been selected and made a part of this new unit. They will be givin extra training specific to the new role they have to perform.