Cant you channel-in the feed/return pipes underground using the new blue plastic stuff (in 15mm) You can insulate properly, and put some isolators at each end!?

Failing that, I'd fit a tiny Combi, cos you then have hot water, too
Of course, you would need a cold supply for that route. If you have a pitch roof, you could stick a header tank up above, but it will be more work and a bit messy!
That way .. you dont "strictly" need a water supply, cos you can manually refill.

The losses are going to be large .. Is this a Single-skin brickwork construction with steel door? Or wooden? Or Sectional concrete?
Roof is what, exactly?

If the size is ample, I'd think about lining it out!

Al.