As Nick said, depends on final product. If you are asking because you are seeking a supplier, the yards I use are fairly local to you.
Apart from oak flooring I buy most of my timber sawn and finish and shape myself.
Unsorted Red softwood from A1 timber near Cricklade
British hardwoods Vastern Timber Royal Wootton Bassett or Tyler Hardwoods, Marlborough
Exotics Yandles Martock
Treated timber for workshops, sheds, chicken coops etc. Webbs Timber, Shrivenham
Bill