I also used https://www.nordictimberbuildings.com/ noted above.
A bespoke 16 x 8 Shed / Summer house combo with extra windows and upgraded roof etc.
Pressure treated, 3 x 2 framework and proper floorboards, couldn’t really find better for the price.
Delivery was £100 or so, and was built on site by them in a couple of hours (included in price)
Lead times might be a while though, guy called Steve was very helpful throughout.