Go - Electrolyte mix for replacing lost salts etc. Particularly useful in warm weather when you sweat more.
PSP22 - Carbohydrate with electrolytes, good for long distances (over 30 miles).
Rego - Protein recovery drink, no use while on the bike but good afterwards.
20 miles a day (I assume 10 miles in the morning, 10 miles in the evening), I would just be using water with a bit of squash providing you are eating sensibly/well during the day to replace some of what you have burnt on the ride.
You would generally use about 60g/70g per bottle, so 500g would make about 7 x 700ml bottles.
The branded powders (like SIS) can get expensive quickly, have a look at this thread which tells you how to home brew for about 25% of the cost.
https://www.cyclechat.net/