NDB Intercepting and tracking is part of the FAA practical test standards.
I dont think holding at an NDB is part of the check ride (I may be wrong, but I didnt do it), but if you know youll be doing it in britain, you can practice it in the states whilst time is cheap.
Any way you look at it, if you plan your training carefully, theres no reason why 15 hours extra won't be more than enough to prepare you for the JAA test.