From the University of Cambridge comes Underground Mathematics which started in 2012 as the Cambridge Mathematics Education Project (CMEP). The site provides a library of rich resources for age 16+ students with the aim of “Enabling all students to explore the connections that underpin mathematics”. Underground Mathematics is being developed by the University of Cambridge, funded by a grant from the UK Department for Education. The resources are free for all users; you can read more about the team and their philosophy here. Their webinar provides an excellent introduction; note the bundle Developing a Mathematical Classroom. Follow Underground Mathematics on Twitter or Facebook.
With the UK new A Level specifications having a greater emphasis on problem solving and more challenging questions, activities such as those here should be a natural part of our regular teaching.
Also from Nrich, see their Prepare for University collection.
For more ideas and resources for this age group see the following slideshow; resources are found from slide 18, the slide images link to the relevant resources.
See also A Level Resources.