It is autumn here, so I'd start now. You need to make a start before the weeds die back and disappear. There's a bit of hard work, but that's why we have gardens!
Search 'glass balustrades' and you will find a lot of ideas that might be suitable. We have friends who have these on first floor decks and they are pretty good, but not as cheap as a plant.
Any chance of a pic?
I can't see what makes this garden 'unusable'.
Young magpies like to hang about together from this time of year onwards.
Yew every time!
I think it might be wise to do a bit of this at a time - 150sq m is quite a lot of rubble. Perhaps try to do the third of the plot nearest the house/in the sun or the least challenging bit. It will give you a sense of achievement.
Get someone in with a mini-digger - not too expensive and very quick.
What do you mean by 'cold storage'?
I'm not so sure that the summer has been all that wet. My garden says not, but I don't know about Weston.