are lights (whatever they are ) needs to be ADR approved for that purpose. ( i dont have a copy of what ADR65- but i think both DRL & parking lights need to comply with ADR for fitments to a road use vehicle)
From what i understand, you can have however have position/parking light fitted - and headlight can remain on
66 Parking lights
(1) A motor vehicle built after June 1953 must be fitted with—
(a) if it is a motortrike with 2 front wheels, other than a
moped, or a motor vehicle with 4 or more wheels—a
pair of parking lights; or
(b) if it is a motorbike with an attached sidecar, or a
motortrike with 1 front wheel, other than a moped—at
least 1 parking light.
(2) A pair of parking lights fitted to a motor vehicle with 4 or
more wheels must be fitted with the centre of each light—
(a) at least 600mm from the centre of the other light; and
(b) not over 510mm from the nearer side of the vehicle.
(3) However, a pair of parking lights fitted to a motor vehicle
under 1300mm wide may be fitted with the centre of each
light not under 400mm from the centre of the other light.
(4) A parking light fitted to a motortrike with 2 front wheels must
not be over 400mm from the nearer side of the vehicle.
(5) A parking light fitted to a motorbike with a sidecar must be
fitted not over 150mm from the side of the sidecar furthest
from the motorbike.
(6) When on, a parking light must—
(a) show a white or yellow light visible 200m from the front
of the motor vehicle; and
(b) not use over 7W.
(7) A parking light fitted to a motor vehicle built after 1969 must
be wired so the parking light is on when a headlight on the
vehicle is on.
A parking light fitted to a sidecar attached to a motorbike
must be wired to operate when a headlight, tail-light or
parking light on the motorbike is on.
Whether these are enforced or not depends on your luck though