They were never very popular, and have disappeared for the most part. This company in the US still sells them, and will ship overseas, but it is likely to be very expensive:
Search at that site for Ridata brand dvds. I couldn't get a direct link to them. They are 9.4gb 8x speed, DVD-R, made by Ritek.
Perhaps someone will chip in with a UK supplier.
By the way, welcome to the forums daveskillin.