It is not a standard product, and as such it could have been made by any suitable fabrication yard or shipyard in UK.
Steelwork, maybe (but not cost-effectively) - control systems and running gear, definitely not. Class supervision to ensure build compliance is standard - just see how many surveyors Lloyds, ABS, DNV etc have in China.
Suspect that a turnkey was required, hence go to a specialist crane manufacturer that can design and build one-offs like this. Babcocks have enough on their plate stitching the ship together, without that. Even Harlands cranes were made in Germany back in the 60s.