Online check-in is hand luggage only and I believe it is 10kg or even unlimited weight per person. It is the size of bag that is restricted.
It only works on the outward journey at Gatwick. You still have to queue and check in, with everyone else for your return flight at Corfu airport. So you will have to get there early to try for a A Boarding Card.
This was how it was last year, however much we waved our Online Check-in confirmation at the Passport Control, they wouldn't let us through. It may be different this year of course.
BBB