Newer apartments don't have a hot water system in the house at all, hot water systems are often centralized for apartment complexes, blocks, or even neighborhoods, and residents just get a bill for their hot water usage. That will make switching over to renewables easier because you can switch thousands of houses seamlessly by making a change in one place, but that isn't what the UK is doing here.

Many older individual apartments that were built before hot water usage was normal use a gas or electric instantaneous water heaters for each appliance or room that needs hot water. Instantaneous hot water systems are a lot more efficient than typical large tank American hot water systems because they don't need to keep water hot when it isn't in use.