That looks like a Lea-Francis radiator, and the wheels, mudguards and runningboards look like the D type. Perhaps this is Lea-Francis car 5388 which was "supplied to Scotland Yard and fitted with a van body of quaint appearance" in 1924 according to Barrie Price (who wrote the definitive book and who's 80 today!).
Oh, and I've just looked at Profile 82 (by Michael Sedgwick) which has this photo, but captioned as a 12/22 although it's actually the earlier 9.8 h.p. D type (as above car number.)