As long as the brickwork is still in a stable condition, then it's no problem. It's just the same as converting a garage.
The main concern is the foundations. Since this would come under Building Control, a Building Inspector might ask for an exploratory hole dug to see the depth and bearing of the footings. Usually it's fine, but you can't assume such.
I would imagine a new roof structure would have to be cut, also.
Building Control would ask for the insulation properties of the wall to be upgraded. Easiest way is to build a timber stud wall all around the inside of the brickwork... preferably with a 50mm cavity to avoid rain penetration. The stud wall would be fitted with insulation.. maybe a combination of ordinary firbeglass or Celotex board, and Multifoil.
Alternatively, the brickwork could be battened off, and Celotex board fixed to that. An onsite talk with the Building Inspector is the way to decide which way to go.
Get Building Control in as early as possible. They're very helpful with advice.