Is it best for me to use the 45A HRC Fuse or a 50A MCB for the shower?
I know the MCB reacts quicker.... but 45A is closer to the current of
the shower... will putting a 50A MCB in slightly overrate the shower?

Use the MCB. It gives much better protection because of the faster
tripping time.. The rating of a fuse or breaker is the current that
the device can carry continuously without tripping. Remember that the
primary purpose of the breaker is to protect the cable. It can't
affect the 'rating' of the shower.

On a 60amp load the 50A typeB MCB will trip sonner than a 45A DRC