Here is a link to a post of Making a Mark, which sums up all the major art competitions in the UK in 2011, with deadlines for entries, dates, tips, things to do and things to avoid, etc. It is divided into UK Major Art Competitions and UK Major Open exhibitions by national art societies.
Apart from this post, Making a Mark is a really interesting blog, 3rd in Top 25 Art Blogs in UK 2010 by Katherine Tyrell, an artist that enjoys wiriting and sharing information about art.
Art competitions are one of the best ways to emerge and be recognised in the industry today. It looks great on your CV and you get to expose your work in major galleries. Now is the perfect time to apply for some Summer and Autumn competitions like The Sunday Times Watercolour Competition or the Jerwood Drawing Prize.