hi there, you should try
www.toptvacademy.co.uk or the production base website. I work for a production company and we use both of these websites to get work experience people. Also look on the bbc website about work experience, they have a lot of placements available but you have to apply a long way in advance. They also do training courses and their website has lots of information generally. Also loads of enthusiasm and a willingness to do anything that anyone asks you is essential. good luck!