Below are a number of useful documents & guides for the controller.

CAP493 Manual of Air Traffic Services (MATS)
CAP413 Radiotelephony Manual
CAP393 Air Navigation Order and Regulations
CAP637 Visual Aids handbook
CAP694 UK Flight Planning guide
CAP717 Collision Avoidance Concepts
CAP745 Aircraft Emergencies – Considerations for Air Traffic Controllers
CAP774 UK Flight information Services (ATSOCAS)
NATS/SRG RT Discipline for Pilots and Controllers
AE GCA agreement AE GCA agreement rev2
AE General introduction to ATC AE General introduction to ATC rev 1
LOAs LOAs between OMAE, OBBB, OIIX, and OEJD rev 3