It's expensive and hard to justify for something that lasts 1 week or so once a year. Everyone talks about doing something to be better prepared but then the heatwave ends and all discussion about it disappears because there's other things to spend that money on.

Expensive to install, especially when your house wasn't built with it in mind in the first place

UK homes are built to the expectation of a mild, damp climate and so have poor insulation and construction requirements compared to places with extremely cold, extremely hot, or extremely variable climates. Furthermore, the government is thoroughly devoted to neoliberalism and has more or less degraded all of its capacity to do anything but support financial capitalism. Climate mitigation engineering is absolutely feasible but not within the bounds of a state that has lost the ability to perform megascale feats like comprehensive grid upgrades and mass scale architectural retrofitting.

Like for instance Canada ran a grant program where the government would just pay for a heat pump or some other home greening measure to reduce energy needs or improve climate resilience. Ran out of money almost instantly because it was so popular, and seeing the huge success of the "give people money to do good things that lessen the ongoing costs of society" program they replaced it with a loan program.

Did the same thing with EV rebates. The rebate programs kept running out of money because so many people wanted to buy EVs, so the solution is to taper off the rebate and end it entirely.

I'd like to think that for many of us, it's because we know how wasteful and harmful it is and would prefer to look for alternative solutions/endure some discomfort for a week or two each year.

Once adoption of AC starts, it will start to be used at inappropriate times, and it will lead to local temperature increases - further driving demand.

I appreciate that for some people and homes, it will be necessary to embrace AC sooner; for the rest, it is critical that this is delayed for as long as possible.

Keep doors and windows closed, darken the rooms with closed curtains. Fans. Wet towel around neck. Cold water.

If I really want to cool down quickly I run my forearm under the tap.

When it's too humid for a fan to cool me off I have these small wearable ice packs that I strap inside of a collar around my neck, and wearing them in front of a fan gets me through the really hot days. Fan is still necessary since you'll sweat a ton when wearing one. The ice lasts about 30 minutes, so you'll need to constantly be swapping packs to recharge, but it's relatively cheap.

there was an interesting thing done during that about how the amount of green space reduced temperature by up to 17°F if I recall correctly

Nah, that’s still way off. London is 51.5 degrees north, which would put it well inside Canada. On the west coast that’s about half way between Seattle and the southern border of Alaska, way up the coast of British Columbia past the northern tip of Vancouver island.

It was about ten degrees warmer at sea level at the peak of that heat dome. It was 37C on the mountaintops, about 1800m above sea level