So far I'm not sure if I've found what I'm looking for (ie ready assemble
home extension kit with no bricklaying plastering and the like), at least
within budget. But, will keep looking... - thanks for the pointers

Now as a concept I like garden offices but I'm not sure most are suitable to
be abutted to a house?. Ecospace though have a very nice one to Building
Stds, but the £20k price tag an issue.

Portacabins and shipping container-type-boxes may have their uses but not, I
would think, abutted to a house as an extension (building regs?). BTW do
portacabins come ready-assembled (a problem with restriced access) or as
packs or sections?.

Timber frame / SIP construction might be a good option in the end, though
cladding in brick or render gets away from the original idea
of just assembly. Will try to find out more.