SS is 3 Cambridge meal replacements a day (or 4 if you're a bloke/ tall woman) totalling 415-554 calories a day,
SS+ is 3 meal replacements a day and a 200cal meal (from the list given in the book) or 4 meal replacements and some skimmed milk. This one comes to 615 calories total.
810, as the name suggests totals 810 calories: so the same 3 meal replacements, and extra 200 calories for the meal (400 rather than 200)- in general the portions are bigger than on SS+ but there are also a couple of extras (like eggs, prawns, chickpeas) on there that you can't have at SS+ level.
Hope that makes sense?
Don't know if you've been on the official website but it's explained quite well there:
Cambridge Weight Plan: Programmes (6 Steps)