what the capacity of 20M of derated 2.5 t&e would be if one side is on insulation for more than 500mm? The other side is clipped to timber at the moment.
A 2.5mm TE x 20m x the whole route touching plasterboard (with insulation) at 30 deg, will allow a 20 amp fuse / load.
If you could confirm that your immersion runs as the emergency HW, rather than x hours per day, that the element is 3000w and the pump no more than 1400w (most run 400-700w) I'd add the pump on.
Mines in the same cupboard as the immersion and the f/spur is beside the immersion 20a switch.
Our immersion gets turned on, for 15 minutes every blue moon.
Maltaron, if you admit that you had to put effort in to 'discovering' what socket and light was on what fuse at the CU, then the original and subsequent installers have not done the works to absolute best practice.
These things do happen and are sometimes the only option. Consider things like tiled, laminate, buried, plastered routes.
Houses with middle divides where front and rear lights and rings incorporate a number of floors are a good example.
When additional works are done some will take the cheap cost option rather than the correct design option. That explains 75% of borrowed landing light neutrals.
Every electrician will endeavour to keep the installation logical, but sometimes it can't always be. Where it can't you have done the right thing and clearly documented the deviations.