There is no need to spend £10k. Have a look on here,https://www.facebook.com/groups/sbcampervans/, there are always discussions on the relative benefits of smaller systems.
The cost of the solar panels has come down, the expensive part are the batteries, I think the
https://www.tesla.com/en_GB/powerwall is about £3k, but really you have to be generating far fair bit of power to make the cost worth while. You can pick up second-hand batteries.
We have neighbours who live off gird, all on secondhand equipment, its not going to be the same as mains but its doable.
I wouldn't go for a shed, I would get a old horsebox.