Pennine boy
You are correct about not knowing what condition the crankshaft is in before the engine Is dismantled but crankcase fretting is a different matter, this is usualy indicated by oil leaks on the case joints or at the cylinder base.
UK industry turnaround time is usualy determined by the component state, some crankshaft & crankcase rework can only be done in the USA and this takes the time, I had a zero defect overhaul done in five days by one shop who I had given about a months notice of the job.
Local re-build vs factory rebuild is not a simple decision with a lot of factors to be considered. But I have never had a UK shop take more than six weeks to overhaul an engine but may be this is because of the volume of business we do and the fact that they know we pay up promptly.