I think it actually depends on the condition of the engine, wether it is good running or in the need of a total rebuilt. I would guess an engine for rebuilt, but complete without damage to the cases and a good timing side could sell for anything between 600 to 1200 GBP ( or more on ebay).
do not underestimate the costs when the engine needs a total rebuilt, Valves, guides, springs, rocker bearings, feedbolts, timing gear camshaft, bigend, barrel, oversize sleeve, exhaust thread repair etc. etc , all those parts and services do sum up!!
I doubt my post will be a great help for you, sorry
Bernd