I was a student in Edinburgh and Cardiff many moons ago and did a lot of windsurfing in both locations. Edinburgh wise Gullane and Longniddrie (30-40min drive) are the two most popular places to sail, but theres no kit hire at either beach. If you get to longniddrie and the winds blowing from the right hand side of the power station then its best to go on to Gullane. Both are bump and jump kind of places with a faint chance of a wave at Gullane. If your after waves you need to go down to Belhaven or Whitesands which are about an hour away. It used to be possible to hire kit from the water sports centre at Port Edgar, but its right under the forth road bridge, gets a strong tidal current and the wind and water quality was pretty awfull so I wouldn't recommend going out there. It gets cold so it'll be 5/3 wet suit, boots, hood and glove weather by then. Boardwise in Edinburgh have just started selling windsurfing kit again and you may be able to blag some trial kit from them if they think you're looking to buy.
I was in Cardiff for a much shorter time but Coldknapp Bay in Barry and Newton Bay in Porthcawl were the two most consistent locations and both could have decent waves on a good day. I can't remmember anywhere that hired out kit but you could call Rat Rigs in Cardiff and they might be able to sort something out. It might be possible to sail in Cardiff Bay in side the barrage now, but it wsn't allowed back in 2004 when I left.
Loads of the Edinburgh locals use this so if people are going sailing chances are it'll be posted on this site ; there's a fairly healthy number of friendly local crew kiting and windy sailing at Gullane, Longniddrie, Silverknowes ( occasionally ) and others places these days.