Be grateful for some advice from the electricians here.

I've been asked by a relative to complete an electrical installation to
a garden shed/outbuilding. The electrician originally asked to do this
job has disappeared after partially completing it. He has connected a
roll of SWA via an isolator, but that is as far as he got.

It's a relatively simple matter of running the SWA to a CU in the
outbuilding and putting in some lights and a couple of sockets. The
problem is that the appliances to be installed in there will use about
6kW of power and I'm not sure that the SWA is rated high enough. The run
is a good 25 metres and I can't tell exactly what size the cable is.

It looks about 4mm but all I can on it is the following: BASEC
ELECTRICAL 600/1000 BS 5467 2017.

Doesn't tell me much, does anyone here know how its size/rating can be
determined?