Organised by freeflashfiction.com, the latest FFF Competition offers a top prize of £150 and a very reasonable entry fee (especially if you pay by BACS from a UK bank account). There are also two highly commended and three shortlist prizes up for grabs.
Note that Free Flash Fiction always accept submissions of flash fiction – if accepted they are published and can be read for free. For general submissions the upper word limit is 500, but for the competitions it is 300 (so don’t overshoot!).