Thanks for the great replies guys one of the things I really love about these forums is that everyone is full of ideas and experiences and particular thanks for the photo!
Camellia Sinensis and Ceanothus? Never even heard of them but now i've read up on them they sound absolutely fascinating; I think for my garden they would't really work but I love the idea of these on an allotment so will definitely keep these in mind.
In terms of foundations I imagine i'm not that far off from my neighbours foundation in places; another issue is with the house i'm in being a few hundred years old I often find buried 'masses' in the garden; big concrete lumps and pipe-working which I have no way of knowing whether it is active or inactive. This might sound over cautious but my neighbour, who has the same problem with buried infrastructure, found that an old abandoned tap buried at the back of his house seems to issue clean fresh water from who knows where! I assume some of these pipes must be main pipes or power lines so i'm wearing of inadvertently breaching something 'active'
That is my main issue with trees and with bay trees specifically; I love my bays in pots and would be encouraged to try a hedge of them (even though i'm in UK zone 8 so it may be a bit rough for them) but I fear some of the things i've read have led me to believe they can be somewhat invasive.
The pictures of asparagus are stunning; I had no idea they created that sort of 'fan' effect; are they productive plants in cool climates?