GB is not a franchisee of easyJet, GB is now fully integrated into easyJet and is known as easyJet.
Duracellslade you will have to purchase all food and drink onboard the aircraft, this should have been communicated to you when your booking was transferred over from BA to easyJet.
You are permitted to take your own food and drink onboard the aircraft and the crew will NOT be offended if you do so, just remember though that you are not allowed to consume your own alcohol onboard the aircraft, only alcohol that you have purchased from the crew.
Also, if bringing food and drink from home, don't forget the 100ml rule regarding liquids, so any drinks will have to be purchased once you have cleared security.
I do just love it when people who are not employees of easyJet offer "advice" to people on here and use the caveate words such as "probably" or "guessing". If you don't know 100% for sure, then don't reply!
Speaking as an easyJet crew member, on long flights such as a Tenerife I would strongly encourage all pax to purchase their own sandwiches at the airport and not rely on onboard catering. Reason for this is because the easyJet management never look at the bigger picture when it comes to long flights and never sufficiently cater them. On an A320 going from LGW to TFS and back, it will have up to around 360 pax travelling on it, yet the company will only load onto that aircraft a maximum of say 60 sandwiches. The management are far too scared that their will be sandwich wastage - so they load a small quantity on the aircraft, but they fail to note CONSTANTLY that crew complain via their catering reports that there were insufficient sandwiches on the aircraft and that they could have sold twice or three times the amount loaded.
And that is why the crew will not be narked at you taking your own stuff, because they know that at least one travelling party will be happy because they will have been sufficiently fed on the flight!