You can check in the night before. The following is quoted on ba.com:
"Customers departing on British Airways scheduled services from the North Terminal at Gatwick can check in, deposit baggage and collect their boarding cards, the day before their flight departs.
Who can use this service?
This service will be available for customers travelling on any British Airways service departing before 15:15 the following day.
Self-Service Check-in details/Self Check-in process:
Early check-in is available only between 16:00 and 22:00 on the day prior to departure for those passengers qualifying as shown above.
All customers using this facility will need to present themselves, their documents and all their checked baggage as though they were about to travel.
They will be asked to use the Self-Service Check-in kiosks at Zone C, and then proceed to drop their bags at the Fast Bag Drop Desks in Zone C.
Those Customers that hold Premier, Gold or Silver Executive Club cards will also be able to check-in at their appropriate Premium Check-in desk in Zone C (although still not prior to 16:00).
Day of Departure process:
Customers using Early Check-in will need to present themselves on the day of departure at their departure gate at least 45 minutes prior to scheduled departure time for Intercontinental services, and 30 minutes prior to departure for passengers travelling to European and Domestic destinations.
Please note: Once checked in, British Airways will not be able to retrieve baggage for customers.
Customers will be required to provide a telephone contact (preferably mobile number) at time of check-in to enable use of the facility.
We will not be able to check-in non-present members of families/groups."
Just typew in early check in in the search ba.com box and it will appear. As far as what can go through security best check the baa web site as I have moved on from check in and am out of the loop as far as what can go through.
Hope this helps
Bluejay