You can check your wheelchair in at the check in and use it to go through security etc. It is hen put in the hold when you get to the plane.
The other option is have the wheelchair checked in at the desk and use the airport assstance staff to take you through security to departure lounge and then onto the plane. My father uses this option as its quicker to get from a to b etc.
My parents frequently fly with Jet2 from Manchester and occasionally E Midlands and as Phaedra says the quality of assistance varies between airports. Parents also find assistance at the destination airport superior, although I guess it depends where you going. Parents also book assistance at time of booking.
Depending on the type of wheelchair there are different rules depended on the battery type if your taking an electric chair. My advice is ring Jet2 assistance team for your requirements or any questons prior to booking (0800 408 5591) oryou can talk to them on social media.