Don't/shouldn't interceptors have 121.5 anyway?
That's what I always learned, too. As long as one carries an active radio... And even if the interceptor doesn't use 121.5, calling on that frequency should somehow clear the mess, or at least initiate the clearing.
Also, I never heard of this requirement to carry an "intercept card", could this be just one more UK-only oddity? Not that it isn't a good idea, though, but there is already such a lot of paraphernalia to be carried...