If the existing cable is not safe to carry 40A or 45A, you have a potential fire hazard. So I advise not to consider using for one/two weeks.
We need to know the CSA of the cable, it would also be good advice to have this circuit inspected and tested, so it has been proved it is fit for continued service, also as it is a shower, RCD protection is highly recommended.
What type of wylex box is it, is fused or MCB? I assume the fact you state existing 32A fuse means it is possible that you do have MCBs installed but the key factor is the cable size, it needs to able to carry the required current safely.
Providing there is no derating of cable and the distance run is less than 39m, it is possibly that 6.00mm2 cable on a 40A device would do, using reference method C.
But if an upgrade is needed 10mm2 cable would be recommended.
Even installing a separate shower unit, would still require the correct cable sizes for both supply and load sides.