I came across this photo on flickr while looking for Home Guard improvised armoured cars. The photo was taken by John Phillopson and is amongst a group of what I assume are a Home Guard re-enactment society.
This vehicle is based on a Triumph Standard 10 and has mock armour on the radiator and windscreen. The weapon is a Boys anti-tank rifle: just the job for taking on a Panzer Mk IV.
As far as I know this is not based on a real HG car but you never know.
In any case, it is a rich source of inspiration.