If it will be too much hassle to pull it out, just leave it be, stick a label on it saying "dead shower feed" or something. There's probably a proper way of doing it. If so someone will be along soon and berate me for putting your life at risk. I think it will be ok though.
Putting the ends in choc block won't hurt. Consider that, some time in the future, somebody will come along to fix a fault. They'll see the loose cable in the CU and think "A wire fallen out. That must be it!"
Safest way it to move the shower live core from the breaker, and put it into the earthing terminal bar in the consumer unit, this method prepares the cable for future earth testing, if the cable is ever reused.
Label the consumer unit accordingly to alert future users.
One of the first tests when commissioning a circuit, is to link live and earth at the consumer unit and measure R1+R2 to prove the earthing.
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