||as a simple statement of "I do not want to do this, but if you really, really want me to, you are going to have to pay more than someone else is currently paying me". I would look for a larger, reputable rebuilder who has a good amount of experience with OHC engines rather than mess with some one-off "guru" who justifies their ability by claiming they build engines for the CEO of Astin Martin... who gives a shit?
Comparing machining prices in the US to those in the UK is probably not going to help much - the big questions are why did the bearings die so quickly, and why did one cylinder bank lose all compression? The cost of the work is immaterial as I'm guessing you would be fairly happy with the transaction if the problems hadn't occurred.
Quick bearing death typically happens from improper clearance, over-loading, or lack of lubrication... improper clearance = his fault since you paid him to assemble the short block. Lack of lubrication fault depends upon who fitted the oil pump, pick up, and pan, and who filled the pan with oil. If that was all done properly, it's really difficult to achieve a lack of lubrication resulting in bearing failure.
Loss of compression occurs from a multitude of areas. Short of it being pistons or rings related, therefor the "guru's" potential problem, otherwise it's most likely not. From reading most of your previous posts, it sounds to me that someone else did the rest of the engine assembly - did you identify the culprit for loss of compression in your engine?