I said it before and I'll repeat it here. We'd put one up in the air for one six hour sortie and it'd take us another three days to get it ready to go again. That's in the days when the equipment was still in production and we were fully supported by the "VOG" system that guaranteed us a replacement part within 24 hours.
Many of the manufacturers have since gone out of business and even those who remain no longer make the original parts. Does anyone happen to have a collection of ancient Post Office relays in the attic to keep that infernal automatic refuelling system going? How about a "FireTec" control unit or two? Some fuel tank units? - every tank had their own dedicated Sec/Ref number. Then there were those dreadfully unreliable amplifiers for the fuel quantity indication system. No doubt she could be navigated by map and pencil aided by a hand held GPS in emergency, so the clockwork GPI and Green Satin don't need to be too reliable. There's probably a few replacement parts for the Smith's MFS lying around in museums somewhere and I know the aircraft can be refuelled manually - we used to do it all the time - but will the UKCAA certify it for a Permit to Fly without having every airworthiness related system system fully serviceable? I don't think so - and I'm only considering the E & I bits that I knew so well. The engines and airframe spares will be just as hard to come by. Admittedly, most of the obsolete equipment could be replaced by modern equivalents, but that requires expensive and hard to certify modifications and in some cases, even the dreaded "Supplementary Type Certificate".
I really do admire the dedication and hard work of those who are working on the Vulcan to the Skies project, but whatever many folks may think, getting 558 back in the air is the easy bit. The really expensive and difficult part of project will be keeping it there.