The OUP & ICCA National Mooting Competition is one of the longest-running in the UK. Their website offers some great resources, including an excellent 'What is Mooting' piece and hints of other places to go for help: books and websites. There is also a archive of moot problems (currently under construction but hopefully up and running again soon...!) As a result of the 2017 final, there are also a couple of useful videos, including one with Lady Justice Hallett's top mooting tips.
The link below will open the external site in a new window: