The OP asks about a combi boiler so presumably no longer has an immersion heater GSM. Her situation is not quite the same as yours.
Aware that the tumble dryer likes to guzzle kWh and with rising prices in mind I decided to compare the cost of my electric oven , hob and microwave. For this I turned to various sites including the Energy Saving Trust. Whilst the EST does not feature an immersion heater, it does indicate that an electric shower of 7,000 to 10,500 watts costs 125% to 350% that of an electric kettle of 3,000 watts - easily evident. (Another site stated that water heated for a shower by electricity is doubled when heated by gas and doubled again when heated by an immersion heater. This is not a dig at you GSM but an exploration of cost.) A smart meter as I suggested will indicate the actual cost of gas to heat water.
Some people collect the initial through put of shower water and use it elsewhere. Others rinse prior to washing with the heating water in the kitchen. In either case it is not necessary to waste it.
The implied cost of a boiling water tap (not on EST) and your thoughts on it are most welcome. Every day’s a school day on GN.