Hi Brian,
I have just found this very interesting site, I am a retired Met Police Superintendent and have just started to collect Met memorabilia as well as my home Force of Beds Constabulary.
I have recently purchased a large number of Luton police, Luton Borough police and Bedfordshire Constabulary helmet and cap badges. Part of this collection are a day and night Luton Borough police helmet plates and a number of cap badges too.
In light of your earlier comments "I would suspect that the Luton County Borough Police Force, 1964 - 1966, would have had helmets sporting the Queen's Crown, which is probably quite scarce, given the short time of the Force's existence", I have added a couple of pictures that support your hypothesis.
Both helmet plates do in fact have the Queens Crown on them.
All the best,
Stewart