Mr Thant wrote:

(There is a rule that all new electrification has to be AC overhead -
both as a standardisation guideline and a health and safety rule -
which may be where the confusion comes from)

Not quite. The ban is new, independent systems with unprotected 3rd
rail, and comes from H&S law about bare conductors in the workplace.
There is an exemption for extentions to existing systems (Southern
region, LU, Merseyside, Glasgow Subway (though I doubt that will be
extended), and protected 3rd rail systems are permitted (eg DLR). There
has been plenty of non-AC overhead since the rule came in, to name a
few, Tyne and Wear Metro (1500V DC overhead); Manchester, Sheffield,
Birmingham, Croydon, Nottingham tramways (all 750 V DC) and DLR (750 V
DC protected 3rd rail). In that time there have also been extentions to
the Southern system (Solent Link, Bournemouth-Weymouth spring to mind)
and the Jubilee Line Extention on LU.

Robin