1970s is too late for imperial cable, which can look like like 4mm.
4mm FTE and 2.5mm FTE both have 1.5mm CPC (earth). The difference is
in Current Carrying Capacity.
Unless the cable is run through insulation or grouped with other
cables or subject to high temperature there is no need to use 4mm
cable for a single 3kW immersion element. Some even wire short
immersion runs in 1.5mm, but most prefer 2.5mm at least when DIYing.
It may have been done purely so it could be converted to a 32A radial
at a future date if the immersion was dispensed with, alternatively
someone perhaps planned a DIY twin 3kW element shower in pumped store,
no boiler. It may have been done purely because someone had some left
over from a previous job - simple as that. Old school sparks if doing
a favour for someone did use somewhat uprated cable so as to provide
"future flexibility" and of course a bigger cable runs cooler and
lasts longer (although 3kW run even 5hrs a day is not going to stress
even 2.5mm much).
So you can use a 2.5mm cable providing the upstream circuit protective
device is 20A or lower. Alternatively you can buy 4mm FTE cut-length
from TLC online, it is not uncommon but due to lower usage can be more
Check the terminals on the immersion element & that a safety stat has
been fitted (they can be changed if the slide-out type). Usually
neutral is a bit charred, Screwfix do 2.5mm Butyl cable in 1m cut
lengths (note pre-cut) quite cheaply if you want to over-engineer or
the cable runs in a warm location (by the tank).