It's always fun playing fantasy gardens, so thanks for letting us share yours!
A few thoughts for you.
I did a quick search on the ferns
website for hedging plants for you. My first search turned up no answers, but from the second the one that stood out was Tree lupin (lupinus arboreus). It wasn't something that sprung to mind, but may actually be worth considering. Fast growing, nitrogen fixing, doesn't mind the seaside, poor soil or (I'm guessing) alkaline soil, has pleasant flowers and is evergreen. The disadvantage may be that it tend to be a bit sprawling, not easily pruned to a neat hedge and short lived. I think it will grow from seed quite quickly. My first thought was holly, but that isn't very quick growing. Ditto yew. Both make nice neat dense hedges given time and pruning...Other plants you could consider are Eleagnus, not native, but nitrogen fixing again. There are selections with improved fruit, which are really quite nice.
Have a think about how big your plum tree will grow. The dwarfing rootstocks are not always that dwarfing, and the tree will want space for it's roots as well. I would think about moving it a bit further away from the wall.
Wildlife will come if you provide habitat and food for them and don't kill them.
A pond doesn't need to be large or even in the ground. I had quite a nice pond in a half barrel in a previous garden. Just make sure whatever gets in can get out, and don't put fish in it. You probably will need some sort of liner or container though.
I think in your area your subsoil will be chalk, so the soil is likely to be alkaline. I wouldn't strip the turf, particularly if there is much recent dog poo under there! You could get a soil testing kit. You can get ones that will give you several soil pH and main NPK fertility measurements for about £25 I think. If you want the grass area, then with the brambles and bindweed likely to come back, I would just mow for a few years on a high setting. You could spot-plant things like cowslips and snowdrops, or just see what arrives. It sounds like you really fancy a wild flower meadow. I think the RHS
gives a simple explanation that may clarify these for you.
Edible perennials: I can recommend Hablitzia tamnoides; a climber with edible leaves like spinach but less bitter, and sprays of white flowers. For me it doesn't grow too well (my soil is very acid) It may find it a bit warm with you, so try it in a shady corner. Good king Henry (Chenopodium bonus henricus) has slightly bitter greens, but I quite like it, and again likes it shady, and is a good ground cover. Perennial kales may be more palatable, and you could also try seakale, which should like your area well. There are several little nurseries specialising in edible perennials in the UK. I like backyard larder
, incredible vegetables
, also chiltern seeds
for growing from seed.
As regards soil, I personally would avoid importing soil as such, you don't know what nasties might come along with it. The turf will try to regrow unless covered with light excluding material. See if you can get hold of Patrick Whitefield's "How to make a forest garden" or Martin Crawford's "creating a forest garden" both are really good on establishment and include tables of useful plants for different situations.
Talk to your neighbours and have a look what they are growing. Many gardeners are happy to give away cuttings and often have orphan plants looking for a home. Now is a good time of year to take cuttings from fruit bushes for example.