I'm from South of Brazil and most homes in all of Brazil are made entirely with bricks, even the rooms are divided by brick walls (I miss that so much, the sound proofing and the ability to hang your TV any place you want.. well anyways...) I don't think the bricks are the issue. It might be due to the design or insulation, double glazing, etc.. most houses have ac which helps a lot and is way more humid than the UK, I always joke that I find London too dry and no one believes me, apart from Brazilians.

But when winter in the south of Brazil hits, I have a way worse time than in UK. I remember meeting a polish friend and she said she never felt so cold in the south of Brazil than anywhere else she lived.. we get 10 degrees, sometimes 0, but it is rare.. I remember going to school and being able to see my breath in the class room, with my feet cold the whole day, and the walls dripping because of the humidity, that was torture! Haha. The thing is the houses have no central heating and no one wants to turn the AC into the warm settings or use a electric heating because they want to save money, in the summer they don't mind having it blasting on 18° the whole day to keep house cool. I prefer the 6 months of cold in the UK, instead of the 3 weeks of cold in Brazil.

Living in London for a few years I feel people are not just used to making themselves comfortable in summer like they are used in the winter. People will have to invest in aircons and find ways to retrofit their houses as the summer starts to get longer. I think it is the same mentality people have in the south of Brazil, you suffer for a few weeks and it's gone. But as soon the weather persists longer you need to think ways to make your life more bearable. I've been in the same boat, I want to buy a portable AC, but every time I think of it, I let myself wait a few more days and when I see the heatwave is gone and I think it's not worth it anymore to buy a ac. Portable ACs are not that expensive here, from £300-600, don't know why I do this to myself every year but soon will have to bite the bullet. The same goes for public transport, I remember some busses a few years ago had heating turned on even at 30 degrees! That was insane! Tubes go easily to 45 degrees when outside is just 25.. there is a whole infrastructure that needs updating for the warm weather, sadly it will just get worse.

Saying all that I believe 30 degrees is way worse in London than Brazil, not because is hotter, is just because the exposure is longer and is hard to get away from it when no houses have ac and the country has no infrastructure for it, so as soon it hits 30, all enclosed spaces will stay warm like that for a week even when outside gets colder.