Just dug out my copy of 14th edition IEE Regs (1976 edition). It states in B23 that voltage drop from the consumer's terminals to any point in the installation should not exceed 2.5% of the declared nominal voltage, ie. 6V drop for a 240V supply. There is no mention of 2.5% plus 1V in the 1976 edition.
15th Edition 1981, also has voltage drop as 2.5%, and I clearly remember using this value when I was in college in the early 80's.
The change to 4% voltage drop appears to have been made in the 16th Edition in 1991, this is the 1st reference I can find in the Editions that I own.
Reading this topic I can see that the UK has some of the tightest voltage tolerances in the world. When loop testing I normally see voltages between 235 to 245V.
