High energy costs could be an incredibly powerful tool to drive changes in the UK housing stock. It's only when energy is expensive that the payback time starts to make sense. That would need sensible and cohesive policy decisions from people with a good understanding of high efficiency housing. Clearly we don't have that.
My 1930s house is a D, so I had an energy contractor round to cost up some improvements. All the obvious cheap-ish ones have already been done. The next ones on the list were things like solar panels which would take 15 years to pay off, solar water heating which would take 30 years and exterior wall insulation which would cost around £40k and take nearly 300 years to pay off! And arguably worse, they couldn't guarantee that sealing up the house wouldn't cause damp problems to emerge since the damp proofing and construction of old buildings can be inconsistent.
Even things like a heat pump would either need an incredibly beefy drop in boiler replacement, costing more than £30k, or I would need to install a hot water tank, previously mentioned insulation, larger radiators and upgrade my pipes - and even then savings would be marginal because electric costs so much more than gas.
Is it worth spending all this money upgrading old housing stock, or is it better spent on things like solar farms, wind farms and hydrogen (or synthetic natgas) generation. Even district heating might easier than trying to retrofit heat pumps to our old leaky houses.
Ultimately the goal is net zero. Insulation helps, all else being equal, but if the energy production is zero carbon then the energy efficiency of the homes is basically irrelevant. Given that, where is it more efficient to spend the money?
Sealing houses up without accounting for moisture can cause issues, but it's not insurmountable. Rather than ventilating through accidental means, you seal the house until ventilation needs to be intentional - and then you can do it using mechanical ventilation with heat recovery - which effectively keeps 80% of the heat in the air and transfers it to incoming fresh air. Equally insulating without moisture management can cause issues - but as long as the insulation is designed with vapour barriers and ventilation in the right place, it's perfectly doable. The problem is that it's very easy to do it wrong, and not realise until years later all the timbers are rotting because of condensation.
I'd argue that (at least in the UK) we have far too many old houses not to deal with them. Space heating accounts for 27% of our national energy consumption. So much energy, which could be useful, is just wasted.
I can't speak for your situation, but the costs you suggest for external wall insulation seem very high - eg https://www.checkatrade.com/blog/cost-guides/external-wall-i... suggests it's more like £6-£20k depending on house size.
Speaking as someone working in the energy industry, we are a very, very long way from zero carbon energy. Renewables only work intermittently - we don't have a solution to fill the gaps in dark winters or calm days. Batteries are at least a couple orders of magnitude too small, pumped hydro needs very specific geography, and all other energy storage methods are unproven or highly inefficient.
The reason costs are high is partially because costs have gone up a lot lately, and I'm in London. Even a new garden fence is going to cost me around £5k. But also, my house would need to have a brick cladding put on after the insulation to get it past planning. There would also be a lot of remedial work required before installing the insulation, such as removing the existing pebbledash render. And I don't even have asbestos to worry about (as far as I know), unlike a lot of other properties of similar age.
I agree we're a long way off net zero, but don't you think spending all this money on marginal improvements to old houses would be better spent on the generation side of things? Personally I can't see a future in the UK that doesn't involve some kind of hydrogen or synthetic natgas generation. We're going to need to solve the generation side anyway.
I had district heating which I had in my last place, which was great and had the footprint and plumbing was comparable to that of a regular gas boiler.
There is definitely room for a lot of improvement in the UK housing stock but every possibility is poisoned by the large numbers of people it would affect. Not all Landlords are fat cats and not all tenants are poor, which is why things move so slowly.
Even in new-builds, the quality of insulation work can be somewhere between shocking and non-existent. What do you do? You need umpteen million new houses and don't have enough skilled tradespeople to be able to fire those that do a crappy job.
I think a much bigger win would be further back where we need to align incentives and opportunities for school leavers to the industries that are short of workers. It seems daft to me that people can choose "Health and Beauty" courses when we already have a million of those and a massive shortage of engineering trades.