That might make sense if you look at fuel costs only, but if you factor in the additional running and captial cost of having BOTH technologies does it still stack up. Very simplified example.
100% gas:
boiler cost £600, installation £1,000
annual fuel cost £600
annual service £50
20 year total £14,600
100% ASHP:
pump £2,000, installation £2,000 (includes hard-standing and piping for external unit)
annual fuel cost £550
annual service £50
20 year total £16,000
Both gas and ASHP:
capital cost £5,600
annual fuel cost £575
annual service £100
20 year total £19,100
Above is very simplified and does not factor in inflation, nor the extra running costs or extra capital costs in integrating the gas and ASHP (e.g. larger TS, more controls, more pumps). Costs I use are for illustrative purposes, you need to plug your own scenario in.
I would suggest you go with gas only, put in a largish TS (e.g. 500l based on your concurrent bath / showers scenario), put in a gas boiler based on DHW recovery times you are willing to put up with (12kWh boiler to 500l TS is approx 2hrs recovery from cold), and use a standard S-plan configuration to run UFH direct from the boiler (no separate buffer). In 20 years time, if gas becomes prohibitively expensive, then revisit the solution based on latest technology. (In reality, our electricity base is derived by burinng gas so increase in gas price also impact electricity price).