Midwest very sorry about the delay I was away and didn't have time before leaving to open up the laptop and gather all the references. Normally I access the forum on my mobile phone. I hope it will be worth the delay!
I've looked at the online version so I could get the context of the previous page, and I think it's changed from your version.
https://assets.publishing.service.g...BR_PDF_AD_M1_2015_with_2016_amendments_V3.pdf
Using the new document, it's quite specific in the guidance now:
View attachment 43910
Looking at the actual regulation 5 in the legislation it does match your quote, but the context is essential here (see underline):
The Building Regulations 2010 - http://www.legislation.gov.uk/uksi/2010/2214/regulation/5
"there is a material change of use where there is a
change in the purposes for which or the circumstances in which a building is used, so that after that change... ...the building, which contains at least one room for residential purposes, contains a greater or lesser number of such rooms than it did previously"
So it specifically only includes changes in what you're using the existing building, rather than changes resulting from building work.
Also the regulation itself excludes extensions specifically
View attachment 43916
Regarding 0.4, again it's changed in the latest version to 0.11
View attachment 43911
Looking at regulation 3
The Building Regulations 2010 - http://www.legislation.gov.uk/uksi/2010/2214/regulation/3
"
(2) An alteration is material for the purposes of these Regulations if the work, or any part of it, would at any stage result—
(a)in a building or controlled service or fitting
not complying with a relevant requirement where previously it did; or
(b)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.
"
So a socket at 300mm doesn't comply, but it would be a material alteration if you moved it down to 200mm. Adding further sockets at an existing non compliant height, to me (non professional) would be a material alteration. However part M above specifically says that the building overall shouldn't be less compliant, so that clarifies matters somewhat.
Let me know if that is unclear - it's certainly clarified my understanding somewhat!