45 amp SB6000 MCBs did exist - but no chance of finding new ones now.
Dodgy used efforts may be available from the usual tat bazaar, but 45A were seldom used and are now exceptionally rare, and therefore grossly overpriced.
Even if you can find one, installing it in the existing consumer unit is a fail, as it has a single RCD for everything, and shoving yet another circuit onto that non-compliant deal only makes a poor situation worse.
A small separate CU for the shower is the only valid option, unless you want to replace the entire thing.
The usual other things such as main bonding size, total load and so on all require consideration.
Dodgy used efforts may be available from the usual tat bazaar, but 45A were seldom used and are now exceptionally rare, and therefore grossly overpriced.
Even if you can find one, installing it in the existing consumer unit is a fail, as it has a single RCD for everything, and shoving yet another circuit onto that non-compliant deal only makes a poor situation worse.
A small separate CU for the shower is the only valid option, unless you want to replace the entire thing.
The usual other things such as main bonding size, total load and so on all require consideration.