I don't think one day with Mr Rosche qualifies anybody as a guru. The best engine builders are those who do it day in and day out.... but somebody who has worked on V8's and 90's engines would be my choice. As for names that is a tough one. If you want a straight rebuild that is simple, if you want improvements that is harder.
Very sound observation, though I didn't specifically recommend him as an engine builder but as a conscientious automotive engineer with decades of experience who works on your car as if it was his own, and that was true before the one day with Paul Rosche.
I would like to ask why you'd recommend someone who works on V8's though? After all, Rover V8's were contemporary to the S38, but whereas I'd happily have a crack at that 60 bhp/litre ex-Buick motor (while not recommending my services to anyone else mind!), I wouldn't go near an S38. There are of course multi-valve V8's from the 90's, but they are also very different engines to the S38; for example BMW and Jaguar V8's are more modern with variable valve timing and alloy blocks with nikasil bore prep.
This is however a very interesting topic and I would be grateful for any names you would be prepared to recommend as high-performance engine builders in the UK.