Hopefully, it still is, unless you wish to feel the long arm of the law, or whatever passes for it nowadays...in the days when leaving side lights on when parked was normal practice.
Highway code, section 249 (Parking at night):-
All vehicles MUST display parking lights when parked on a road or a lay-by on a road with a speed limit greater than 30 mph (48 km/h).
[Law RVLR reg 24]
250
Cars, goods vehicles not exceeding 1525 kg unladen weight, invalid carriages, motorcycles and pedal cycles may be parked without lights on a road (or lay-by) with a speed limit of 30 mph (48 km/h) or less if they are
at least 10 metres (32 feet) away from any junction, close to the kerb and facing in the direction of the traffic flow
in a recognised parking place or lay-by
Other vehicles and trailers, and all vehicles with projecting loads, MUST NOT be left on a road at night without lights.
[Laws RVLR reg 24 & CUR reg 82(7)]
