I have an idea just want to hear what others say.
The timber will be unpainted but probably stained, perhaps oiled?
I'm in the UK, South West, Swindon area.
Seeing that you are from the UK these would be my picks....
Hardwoods that would be "desireable" for your use would be White Oak (American), the Mahoganies (Phillipine, Honduran), Locust, Ipe (iron woods), Mesquite, Ash, Teak and Eucalyptus...
Softwoods would include Western Red Cedar, Redwood, Port Orford Cedar, Southern Yellow Pine, most Spruces, and Bald Cypress...FWIW.
Best picks in order would be...
Northern White and Western Red Cedars..
White Oak... Northern White and Western Red Cedars
Both of these woods are native to North America and are traditionally used for boat building, house siding and furniture. They’re valued for their combination of lightweight, interesting grain pattern and extreme durability in outdoor conditions. Although cedar will show knots and cracks in the grain, it maintains its durability for 20 years or more without warping, splitting or rotting. Its light-colored surface will weather to a silver-gray patina.... Teak
The king of durable woods, teak will hold up long enough to pass down to the next generation. It needs no maintenance (aside from the occasional light sanding or cleaning to remove surface dirt), is dense and straight-grained, and will not warp or crack over time. Because of its high mineral content, teak resists rotting even in the wettest conditions. Over time, the surface of the wood will weather to a beautiful silver-gray patina... Eucalyptus
A renewable resource - eucalyptus is a plantation-grown hardwood that is sustainably harvested and in plentiful supply. This high-quality, kiln-dried timber is incredibly solid with great durability and strength and has beautiful grain and a smooth finish that requires minimal maintenance. Eucalyptus is extremely dense, rot- and decay-resistant with a high oil content that repels water and moisture. It also weathers to a soft gray if left untreated, however, it can be stained to maintain its rich tones.... American White Oak
Its unique cell structure repels moisture, insects and rot. The famous American sailing ship, Old Iron Side, was built with white oak and could repel British cannon balls. Dense and straight-grained, white oak furniture has an oil finish and can be left to weather to a gray patina or cleaned and re-oiled annually....
But you know there are more choices...