I don't think it's possible. but, I do like a challenge! Remove the bath panel and the bath legs, see if the sealant around the bath holds the bath up. If so multitool around the edges of the rotten floor. If you angle the blade of the multitool sideways you should be able to do it, it'll be tricky and annoying with the restricted space but possibly could be done. Then pva the edges of the floor that was rotting (they would still be wet and rotting and don't want that moisture passing into the new floor and it should just dry out if not too damaged already. Replace then floor with chipboard. You might have to do it in 2 halves if when you remove the legs the bath starts to drop, so maybe remove 1 leg do that bit of floor, then the next. Fuck knows, worth a try.

But it most likely is the neutral in the lamp holder, buy a new pendant and switch. Turn off the power, remove the blue and brown flex but leave the rose and all other wiring in place, take the pendant off the rose of the new light and connect the blue and brown into same terminals as before (you'll need slide the old rose cover over the new pendant flex 1st) and change the switch. If that doesn't work definitely contact an electrician.

Electrician here. It looks to me like the neutrals (blacks on the left with the blue flex to the pendant) have come out and need re-terminating, the switch would be live and operate as normal but the light needs a neutral to work. The only problem would be that op said no other lights had stopped working, as this isn't end of line (last light on the circuit) the neutrals not being connected would effect any other light further down the circuit. Test between the earth and switched line (the black with red sleeving connected to the brown) And check for voltage, then check between the neutral and switched line. If all reads 230v the problem will be in the lamp holder (the bit the bulb goes into) if you don't get 230v between switched line and neutral (possibly 115v or something similar) check between neutral and earth if you have 230v between the 2 you have a neutral missing.(just to be clear neutral is a live cable) If you don't have 230v between switched line and earth the cable between the switch and pendant is probably damaged or poorly terminated in the switch. But in all honesty, I recommend contacting an electrician. The equipment you're using to test could be dangerous and not fit for testing this type of voltage. And please do not use the little clear screwdrivers with a light on top, they definitely are dangerous!