With no RCD, nobody should sign a BS7671 certificate or self certify it to the LABC under Part P - they would be effectively committing fraud if they did so, since there is no way that work complies with BS7671 or Part P.
As has been said, what should be done is the tails should be split, and a separate mini CU, with an RCD and MCB in for the shower should be installed. That can then be tested regardless of the state of your existing CU (which incidentally I would recommend you get replaced at some point anyway, but that's a separate matter...). Hopefully the new cable is long enough that it can reach a separate CU, it might be possible to extend it if it isn't, although that's not really a nice thing to do on a cable of that size...
As has been said, what should be done is the tails should be split, and a separate mini CU, with an RCD and MCB in for the shower should be installed. That can then be tested regardless of the state of your existing CU (which incidentally I would recommend you get replaced at some point anyway, but that's a separate matter...). Hopefully the new cable is long enough that it can reach a separate CU, it might be possible to extend it if it isn't, although that's not really a nice thing to do on a cable of that size...