TT,
Under the UK ANO, pilots flying at not more than 3,000 ft are deemed to be complying with IFR provided they remain clear of cloud and in sight of the surface.
Alternatively, fly at least 1,000 ft above the nearest obstacle within 5 nm if you are able to fly IMC and choose that option.
Remember that the usual low flying rules still work at night....
In addition, fly quadrantals above 3,000 ft amsl (or above the transition level if higher).
N.B. The police may fly under PAOC "Visual Contact Flight" rules at night.