It's all been said really but I'll say my bit anyway
I had the same question for Stroma recently for a rewire. Stroma said I needed to have all sockets/switches at the building regs heights. I decided not to due to the following:
BS7672 only says the socket has to be mounted at a suitable height so it will not be damaged.
Part M of the building regs does indeed give certain heights, however, Section 0, 0.2 states that
"The recommendations of this volume of this approved document apply to newly erected dwellings, and dwellings undergoing material alteration, only. They do not apply to the extension of a dwelling."
So a rewire cannot be classed as a newly erected dwelling but the building inspector may say it is a 'material alteration' (even though it's not). Here's what you can then say:
Part M, Volume 1, Section 0, 0.11
If it is a material alteration then the dwelling should be 'no less compliant'. Therefore using existing heights will be ok (as long as they meet BS7671 553.1.6).
Also, The definition of a material alteration is found in the
Building regulations (2010), Part 2, Regulation 3(1)c and 3(2) a and b:
A material alteration is an alteration where the work, or any part of it, would at any stage result:
"In a building or controlled service or fitting not complying with a relevant requirement where previously it did, or
In a building or controlled service or fitting which before the work commenced did not comply with a relevant requirement, being
more unsatisfactory in relation to such a requirement."
I'm ready for any inspector!!