Probably not. You'll need to ask the vets what they mean, but they likely mean a fit to fly statement/certificate. The EC 998 form is the import paperwork your pets need to be allowed into the UK, but does not certify their current health in any way (just their past history of having been microchipped and given rabies/tapeworm/tick/flea treatment, as appropriate).
Depending on when you fly, some airlines also want extra documentation (acclimation for temperatures in the extreme heat/cold, for example).