Replacing an immersion heater is no more difficult (apart possibly from the amount of physical effort involved) than changing a 13 Amp plug.
All that needs to be done is to drain the tank, turn the power off, disconnect the wires, unscrew the existing immersion heater (using a special spanner, which any tool hire shop can provide), screw the new one in and connect the wires up again. The actual heater only costs about £15 to £25 and minimal labour is required. (It's only a few minutes work).
The only potential hazard is that, in seeking to ensure that the new heater is screwed in really tightly, it's possible to crack the hot water tank at the point where the immersion heater goes into it. If that happens the crack can simply be soldered up quite easily. (Soldering is meant to be only a temporary fix until a new tank can be fitted but I cracked my own hot water tank, while screwing in a new immersion heater, about 20 years ago and the solder is still holding firm on it now).
Just like changing a 13 Amp plig, there's no 'registration', or other paperwork, involved.