Will you be moving them around? Plastic is light and easy to move; HDPE is a bit heavier; wood is heavier still. Does the chair come apart to make it easier to get through a door / store in a garage over winter? Wood is lovely, but takes some maintanence - it needs occasional sealing. Do you want something that's adjustable or fixed? Lots of adirondack chairs can be low to the ground, so you want it to feel safe for old people who need to heave themselves up. Are you happy with self-assembly? How much self assembly?
B&Q / Domu Brands £105. This chair appears to be available in griege (or "aesthetic"). I don't know if other colours are available. It folds, but that folded item looks a bit cumbersome. The front edge (the bit under your knees) looks a bit square - I don't know if this would be painful. It doesn't come with a footstall. Vonhaus sold via B&Q
They sell a non-folding version. The leading edge (the bit under your knee) looks to be a bit more rounded, and it's available in a nice (mint?) green. Vonhous folding adirondack sold via B&Q
These chairs are expensive, (£350!!) but they don't need assembly and they come in a wide range of nice bright colours. They're made out of recycled plastic but the company is upfront and honest and doesn't use weird "plastic wood" lies to describe the material. The back has nice countours, and I feel like smaller slats for the seat combined with better rounded edges will be comfortable. Dark colours may get a bit scorchio in direct summer sun. They offer free delivery. https://www.mobek.co.uk/product/c01-classic-adirondack/ They also offer a range of footstools which are about £100 - £150 each. This does bump the cost up quite a lot. I guess you'd look at this as a lifetime investment though. I don't know if Mobek are re-selling the same chairs as this other company: https://www.greatgardenproducts.co.uk/product-category/garden-furniture/adirondack-chairs/
For something a bit cheaper, (£115 in the sale?) that needs self assembly, there's the Bjorn adirondak. They call it synthetic wood, it's HDPE plastic. Bjorn adirondack sold by BillyOh
Dunelm sell a nice chair and footstool made from actual wood. This is just £99. They describe it as weather resistant. I don't know if that means they've got hidden instructions in the manual saying "bring it inside if it rains" which would not be fun. Vonhaus via vonhaus
Robert Dyas have a pressure treated adirondack chair. It's £160, or £115 in the sale? They say "it will look green because it's pressure treated" and also, in bold "It's been pressure treated so of course it's green, you can't return it just because it's green, the green will fade" which I think is quite funny. If you want something to fade into a natural grey colour these will do that. Do read the reviews though. Robert Dyas
Sustainable Furniture have something called Swedish Redwood Adirondack chair. The chair by itself is £175. With a footstall it's £215. Redwood adirondack chair
If you're pushing the boat out and want teak these are £500, or £350 in a sale? That includes free delivery too. I'm not sure if the foot stool is removable or not, and if you do remove it I don't know how comfy they'd be. Garden Benches Teak adirondack
Again at the higher end, these use cedar, are customisable, and they do look comfy. £475 (but with cheaper options available). Cedar adirondack chairs