Sorry if someone else has already pointed this out but if they have, I haven't spotted it.
My local Wickes has piles of half-price timber around the place. When I asked the blokey, he said they are changing the range (which seems a bit odd) but presumably that means all their branches are flogging stuff off.
I went to buy some gravel boards to lash up some low raised beds in the garden and came away with treated redwood at a much lower price (£3.30 for each 2.4m length of 144 x 20.5).
Worth investigating if you need anything as there seemed to be quite a mix of types and sizes but I would recommend taking a saw as many pieces are 3.6m long, so wouldn't fit in a car.