You forgot to mention electric plugs. UK is the home of the world's finest plug and socket system. However they are rather big.
The reason for the separate taps is that Britain was industrialised and had running water in towns and cities earlier than most countries.
In my lifetime the last of the old Roman waterpipes in London were put out of use, there may still be some working in places like Bath. The oldest piped water in a semi-domestic residence, still working, in England, was installed about 1530 in the king's private apartments, to reduce the risk of infection or poisoning. The supply was maintained in secret until recently.
For the next two or three hundred years most of the pipes were lead, which is a soft metal, and they were of small diameter. The water pressure was quite low, and, as still happens in some countries, the supply was intermittent.
The system was therefore designed so that most houses had a water tank high up, usually in the roof, which was filled at low speed and stored sufficient water to supply the house at times of day when the supply was poor and demand was high.
Drinking water was delivered direct from the mains to a tap in the kitchen, and was protected from contamination. From 1854 in London, the risks of contaminated water from pumps and wells led to a reduction in their popularity.
Hot water was heated in various ways, and delivered, at quite high flow rates, from the tank, so that a bath could be filled in a matter of minutes (faster than the main delivered water to the house).
This meant that the water pressure in the hot and cold pipes was different, so even when mixer taps were invented, later, they were unsuitable because the high-pressure water from one pipe could force its way into the low-pressure pipes, with various undesirable results.
At no time in the following 300 years did it seem like a good time to change the plumbing system in every house, every street and every pumping station to a high-flow, high-pressure system. Apart from the cost, applying high pressure to old pipes tends to make them burst, and to old taps and fittings tends to make them leak.
You will be delighted to learn that a hygienic design of mixer taps was introduced to British kitchens about 60 years ago, with features to prevent mixing in the pipes (modern mixer taps are actually not as good in this regard) and over the last 40 years, the old water mains were mostly been replaced with steel and plastic ones that can accommodate higher pressure and flow. As new houses are built and old ones replumbed, they are fitted out to use high-pressure taps, including Italian ones, and mixers. Many of them are still slower to fill a bath than the earlier plumbing system, though.
I will be pleased to expand on this brief summary if you wish.