If you come at this another way you will get a better idea.
1. how many WG's in the UK
2. average take by each girl
3. total number of punts
4. average number of punts by each punter
5. percentage of eligible men who are punting
Each of these will just be a guess but you can be fairly sure your guess is within a factor of 3 or so.
For example an average punter wont do ten punts per month nor one punt per year. Ten per year seems believable.
You will find I think, that it takes about one bloke in ten to be an average punter to keep all the AW, parlour and street girls busy.
Think about your mates, work collegues, relatives etc is it believable that ten per cent are punting, Seems OK to me.
This produces an enormous amount being spent on punting. The good old gov said it was 6 BN.
And when they did this the EEC wanted more VAT.