Most garages won't be interested in doing a job like that, they make far more money doing quick turn around jobs like servicing brakes and exhausts.
That said there will be someone out there who will do it, but if you think that £600 is very expensive then I think your being unrealistic especially if that was including fluids and sundries, Dan the man in a driveway may do it cheaper, but £600 represents about 10 to 12 hours labour inc vat in your average independent garage, which may be okay for a like for like swap but for anything other than that there will be some degree of modification required, some more involved than others and these are the things that take the time.
A defender 200tdi transplant is the simplest solutionIf you go for a 300tdi or V8 then there is alot more to it and you have to fabricate mounts, modify wiring exhausts possibly change gearboxes etc etc and may be better with a donor vehicle, so I'd expect you'll be much more than £600 labour.
