Bampton's wood yard, Vespasian Road Southampton. They do a good range of tropical hardwoods, including teak, and will cut it to size at a reasonable cost. They also do budles of offcuts, which for what you want sounds like they might be worth a visit.
Heigham Saw Mills in Heigham street Norwich... They also do a decent line in PNG rosewood a great substitute especially for such as duckboards and at a fraction of the price of Teak but with all the salt water durability.