The following are websites devoted to mathematical problem solving, and are an excellent resource for problems of the style covered in the enrichment programmes.
IMO problem archive (advanced problems)
The following books are good resources for problems as well as problem-solving techniques.
The IMO Compendium by Djukić, Janković, Matić and Petrović; a very large collection of IMO-related (shortlisted and longlisted) problems.
Problem Solving Strategies by Arthur Engel; a problem-based tour through all problem categories.
Euclidean Geometry in Mathematical Olympiads by Evan Chen; an insightful problem-solving book in Euclidean geometry written by an IMO Gold-medallist.
Geometry Revisited by Coxeter and Greitzer; an excellent and thorough treatment of elementary geometry.