Is it true that modern UK building regs forbid the construction/excavation of new cellars?
Not true. You need planning permission to construct a cellar, primarily because you are increasing the floorspace of the property. This is quite a different thing from Building Regs approval which you also need – as you do for anything which might affect the structure, or the health and well-being of the occupants. Always a lot of confusion on this point – but there is a clear technical distinction between planning which is civil law and Building Regs which is criminal law. (meaning you can be jailed or fined for offences under the latter, but not the former). Most utilities do not run under houses except for private drains which can usually be diverted. In central London there are tunnels and tube lines and a load of other things like WW2 citadels but these are so deep they are not affected by relatively shallow basements. London is constructed on clay and gravel and the formation of new basements is a viable solution particularly where there are other restrictions such as building in
