Be aware that UK water regs prohibit amateur installation of unvented
DHW systems and that building regs approval is required.

My understanding that it is an "or", not an "and".

i.e.

Be aware that UK building regs prohibit amateur installation of unvented
DHW systems unless building regs approval is obtained.

Christian.

P.S. it is only above a certain storage capacity that this applies. (For
some reason 13 litres rings a bell, but I could be well out). This enables
unvented mains pressure water systems not using storage to be fitted without
approval (unless also required by Part L).