BBC weather site has them, in the Coast and Sea section. Also has helpful little graphs! Where are you thinking of going? I used to ride at Fairbourne a lot, if you're going there or to any of the beaches along that coast you need to go right at low tide, or there isn't any beach!
it will give times of nearest place to it.you need to add or subtract so many minutes per mile it is or somthing like that depending on which direction away you are from nearest place mentioned.