Avantix shows Peterborough-London Bridge, route FCC only, cheap day return for 4 adults with group save 4 discount applied. This is correctly the coast of 2 adult cheap day returns. This ticket is valid leaving Peterborough at or after 08:45 and returning isn't valid on trains scehduled to depart or pass through King Cross or St Pancras between 16:30-19:01 inclusive.
If you want to return on a different day, you want a saver return, which with groupsave 4 is £71, which is valid departing Peterborough at or after 09:15 and returning after 09:30 apart from trains booked to depart or pass through Kings Cross or St Pancras between 17:30-18:30.
If you want an unrestricted ticket, this will cost £94 for an open return with groupsave 4.