If I buy a Return Cambridge-Isleworth I can't use it on the 8th when I travel from Heathrow, right?
There are three types of Return through tickets available between Cambridge and Isleworth: the Anytime Day Return, the Off-Peak Day Return and the Off-Peak Return.
If your travel times suit the restrictions applying to the Off-Peak Return (which are Monday-Friday morning peak restrictions, plus a one hour evening peak restriction out of London) then you can use just one ticket, namely that ticket, for all your travel as the return portion is valid for a calendar month from the first day of validity.
There is no Anytime Return, and accordingly, if you require travel during the times when the Off-Peak Return is not valid then you will have to travel using something like two Anytime Day Singles. Alternatively you could obtain an Anytime Return between Cambridge and Heathrow Rail, (which is valid for two days on the outward portion) but at £106.20 this is not a cheap option!