My recommendation was for the american hardwoods sold by Moss and Co in Hammersmith. They sell everything from american white oak (much lighter in colour than english oak, through to black walnut which is black but very dark. It's all kiln dried and, uinlike english hardwoods, is straight edged and the planks are reasonably flat meaning that there is a lot less wastage. I'm waiting for a new pricelist from them, but because of the less wastage than buying waney edged stuff, they are quite economical
Their phone number is 0208 748 8251. They are in a side lane running back from 104 King Street, Hammersmith. They can deliver stuff but you really need to go yourself to pick out what you want, which is easy because the planks are stored on end and you can sort through them easily
No connection with them other than as a satisfied customer for a good many years
John