It is stamped 1940 with an unclear makers mark but it appears likely to be BMB - Briggs Motor Bodies of Dagenham. There is also what appears to be a period canvas chinstrap.
Superb 1940 dated WW2 British Brodie Steel Helmet
GENUINE WW1 BRITISH ARMY BRODIE STEEL HELMET LINER RUBBER DOUGHNUT RING
Genuine British Army Brodie Steel Helmet
Original British Army WW2 Combat Steel Helmet
WW2 British Brodie Helmet 1940 Dated TTC Steel Helmet Camo Net Textured Finish