Decarbonising heat with hydrogen

The current UK strategy is to carry out research and development into hydrogen heating during the 2020s, with a view to commercial roll-out during the 2030s and 2040s. There are practical reasons for this, which politics cannot change:

Hydrogen heating is an immature technology, which has not been deployed commercially at scale anywhere in the world.
Hydrogen manufacture is more established, but mostly for niche industrial uses, and needs significant development for commercial scale-up, including a strategic view of the availability and security of the necessary resources.
The main technology, which is expected to remain the main technology, for producing hydrogen is cracking hydrocarbons (primarily natural gas). This is obviously not a low-carbon solution unless the carbon is captured. The credibility of heat decarbonisation through hydrogen heating depends on very large scale Carbon Capture and Storage, which is also immature and commercially unproven.
As all UK parties are currently opposed to fracking and UK Continental Shelf resources are drying up, hydrogen production by cracking hydrocarbons will mean increasing dependence on imported fossil fuels.
Heat is a very seasonal demand. The technologies that would be used to produce the carbon (whether cracking hydrocarbons or electrolysis using low-carbon electricity) do not produce to the same seasonal pattern. The technology therefore relies either on highly inefficient operation of production facilities (e.g. mothballed for half the year) or massive seasonal storage of the hydrogen. Hydrogen heating therefore needs two forms of massive gas storage - for carbon capture and seasonal hydrogen storage. None of which is currently available.
This also restricts the locations in which hydrogen can be produced to where the facility can be connected to two separate huge, long-term storage facilities (typically geological features such as salt caverns or depleted gas fields).
The geological gas storage engineering has to be developed to guarantee minimal long-term leakage and no geological impacts (e.g. subsidence/fracturing).
One could not convert a gas network to hydrogen until every single boiler and industrial user on the network were able to handle hydrogen. Simply swapping-out the boilers in readiness for a conversion to hydrogen is probably a decade-long task. And that assumes that people are willing to gamble on spending the money on replacing every unit with a dual-fuel (methane/hydrogen) system before the viability of hydrogen heating has been demonstrated. In all likelihood, one wouldn't even start the mass equipment switch until late in the next decade once the necessary technologies were proven.

This all has to be done at much larger scale than envisaged for the electricity system, because heat demand is double electricity demand, and at much lower cost, because people are used to (and seem to expect) heating fuel prices that are around one-third of the cost of electricity per unit of energy.

The UK currently produces 27 TWh of hydrogen p.a. which would have to increase more than tenfold for hydrogen to supply half of the UK's heat, even assuming hydrogen plays no role in transport and electricity. (CCC 2018, p.19)

The current retail price for hydrogen is around £10/kg. At a Gross Calorific Value of 39.4 kWh/kg or Net Calorific Value of 33.3 kWh/kg, that equates to around £250 - 300 /MWh. That compares with around £46/MWh (4.6p/kWh) for domestic gas.

That is "brown" hydrogen produced by the cheapest techniques (cracking hydrocarbons) without Carbon Capture and Storage and seasonal hydrogen storage, which will both increase the cost significantly.

Some radical advances down the learning curve and economies of scale have to occur before hydrogen could be rolled out as a heating fuel without huge costs to consumers or taxpayers.

References

Committee on Climate Change, Hydrogen in a Low-Carbon Economy (Nov 2018) https://www.theccc.org.uk/wp-content/uploads/2018/11/H2-report-draft-20181119-FINALV3.pdf

"How much does a hydrogen car cost to run", Evening Standard, 25/7/2017 https://www.standard.co.uk/lifestyle/motors/how-much-does-a-hydrogen-car-cost-to-run-a3595841.html

Ofgem, Infographic: Bills, prices and profits (Nov 2019) https://www.ofgem.gov.uk/publications-and-updates/infographic-bills-prices-and-profits