Hello James. This may sound odd, but have you considered what you want, and then going with that? For the sort of numbers being bandied about, I would be throwing that right into an extension.
Cash tight because many, many mortgage? Maybe practicing laying track, modelling, whatever. How's that BLT idea coming along? Many ways to skin a railway cat. We don't do that to cats, btw, that is for another modelrailway 'forum'. We would give them a uniform. Or a hat. More practically, somewhere comfy to sit where their fur is far away from gearboxes and stuff.
They way things are going, 'Our Glorious Leader' will nick your garden and build a flammable multi-occupancy 'dwelling' anyway. And then up your CT with reduced bin collection/dibble/useful things.
OT your shed will be worthless in a land valuable area, when it comes to selling time.
Kindest regards, from someone who had big ideas in London and now cannot wait to sell up and b#%%{r off.
Bernie
ps nice garden, btw. It looks like a pleasant place to be in the big smoke.