BHA Information PDF/Leaflets took the option of remaining 'out of date' until they are back 'in date' again
As long as you have planning permission sorted - as a purposed elevated helipad would so require - you can do what you like (within a large degree) - assuming you have clear approach/departure paths and uncongested etc there are various publications for AOC use buy also contain some hard-earned 'lessons' you might like to employ in your own design. Web search UK Hospital Helipad Design CAP etc etc