This is definitley not a VG 500 twin. The exhaust pipe on the left hand side confuses the issue somewhat but identifies it as a 350cc NH Red Hunter of the late 40's-1950 period.
For some reason the Ariel factory at that time fitted the NH with a prewar style twin exhaust port head whereas the equivalent 500cc VH Red Hunter had a much more efficient single port head. This was an annoyance to NH owners as it doubled the cost of a replacement exhaust system so you paid more (a lot more) whilst knowing the twin port head reduced performance. This was on top of only being a 350 when all your mates rode 500's or even the occassional 650 twin. As you can guess, I had one when it was old and weary and I was young and broke. It was a very sweet bike but like all teenagers, sweetness was low on my list. I was not who the bike was intended for.
Cheers,