As others have said, changing this to a garden will likely need planning permission.
On the other hand though...
"I’d like an orchard, pond, wild flower meadow etc."
That is entirely doable. Also, since last year (the rules changed) you can now get help from the government with some of these things.
It used to be restricted to land that was 5 hectares (12.3 acres) or more. But last year that rule was scrapped and now anybody who manages land can apply for a government grant under the Sustainable Farming Incentive scheme (SFI).
You can register your land as permanent grassland for the SFI and then apply for grants to help you manage the land.
https://www.gov.uk/government/publications/sustainable-farming-incentive-scheme-expanded-offer-for-2024/sfi-scheme-information-expanded-offer-for-2024
This includes things like putting in a herbal ley (as a pp said), which is effectively a wild flower meadow. The government will then pay you £382 per hectare (2.47 acres) per year for three years.
If you have a pond on your land that is at least 25 sq metres (5m x 5m) then you can get £257 per year for three years to maintain it.
With an orchard, it may be worthwhile growing it as a shelter belt where you've currently just got a hedge, so around the boundary of the property.
If you do want to create a bit of woodland on your property then you can certainly do that without planning permission but you won't get any grants unless you've got at least 5 hectares of land.
Apart from that, you will likely be able to extend your garden proper, somewhat.
It likely won't be noticed if you extend your garden to some extent but you certainly won't be able to turn the whole five acres into your new back garden without planning permission.