The liners your looking at for that sort of money must be PVC. I have one in my pond in the old house and they are very rigid and a nightmare to fit. You want to be looking at rubber butyl that is much easier to work with but gonna be around £3-£400 Maybe more. But worth it for the finish. I am awaiting a box liner quote myself and may go that route myself on a 20ftX15ftX5-6FT new build.
“The only place you will ever find success without hard graft is in the dictionary”