Bad as the Iranian regime is, we know that foreign governments are actively working for regime change/collapse in Iran (Mossad boasted in public about being with the protesters on the ground in Iran, whether that's true or not it seems like a statement intended to make things worse). So maybe be extra careful where you source those death numbers claims.

UK is not, and will not be in the situation Iran is in for the foreseeable future. There will not be several powerful countries, some widely hated in UK and openly preferring a UK in smoking ruins to democratic government in the UK, calling for revolution there (although don't get me wrong, UK too could totally could use a revolution). UK has nuclear weapons. UK has a world-class surveillance apparatus, and doesn't have to contend with the cynical people running it getting regularly murdered or bought out by more powerful actors.

What all this means - and this has been the core message of just about all dissidents in western countries for decades - is that the people with control in the UK don't have to gun down hundreds (or tens of thousands, if you believe the colored reports) in the streets to cling to power. If it was their best option, they might.