Replacement for my old garage

Scaffold frame, about £1 per foot and £2 per clip.
Build the walls with pallets. Securing them to the frame by putting the frame inside them.
Scaffold bracing across the floor, close enough to put pallets down on.
Cover the outside and floor in more wood rescued from pallets.
Run conduit through the pallets inside, before packing with insulation material.
Line the pallets inside.
You now have a solid structure that can have shelves etc easily attached, and is easily extendable whenever you choose.
Paint with creosote or paint as you see fit.

£120 Length 3 x 15 each side, 2 more for the floor
£56 Upright 3 x 8 height each side + 1 rear
£51 Apex 1 x 15 for apex, 3 x 6 each side to make the triangle
£30 Back wall 3 x 10 across, floor+ mid + top
£20 Front 2 x 10 across floor + top
Plus clips.
Plus feet
Make doors

A couple of scaffolders could build that within 2 hours if you don't want to do it yourself. Have a word with a team if you see them whilst out.

Prefab concrete should last longer.
What's wrong with your existing? Can it be repair? (edited)