There are three trades in the Corps of Royal Engineers that will accept you without any GCSEs: Armoured Engineer, Driver and Signaller. Like everyone else in the Corps, you will also be trained as a Combat Engineer (building bridges, clearing minefields, setting up field fortifications etc etc).
If you want the non-party line on these three trades: as an Armoured Engineer you will play with nice big toys whilst improving your pie-eating and beer-drinking skills. You are also likely to spend a fair while in Germany working on your porn skills. As a Driver RE you will be employed primarily in an MT (Motor Transport) Troop. You will be the butt of all tradesmens' jokes for being a thick cnut but will have the last laugh when you're toasty in your cab and they are shovelling mud in the rain. As a Signaller you will listen to beeps all night and be too close to the Badge (Sergeant Major) and OC (Officer Commanding) for comfort. You will probably also develop the urge to read comics.