there are hundreds of places in the uk to buy it all from and to be honest a lot of them are around the same price... ebay is goin be your cheapest option unless you can find someone to sell you one...
if your in doubt mate pop the link in one of the better growers loft and ask is this any good? were all here for the same reason so theres no reason why they would ignore you!
Just be carefull with flea-bay, lots of fake and substandard unsafe gear is sold on there, make sure the ballast is BS kitemarked and carries the CE stamp.
Remember safety first, especially if the grow is in your home.
Its not worth saving a few quid buying unsafe gear, not saying you cant get quality stuff on e-gay, just be carefull and if a deal seems to good to be true it probably is, just that.
And dont forget to get a relay/contactor if you wanna run a 250w or higher HID in conjunction with a timer.