Okay onto the next problem the AC coming into the cab is incomplete and I'm not sure what's missing. Photos bellow. The first picture is the power cable coming from the monitor I have no where for this to go. The second is the power coming into the cab ends at this connector.
I don't think it was mentioned yet, in UK they have different standard AC wire colors than in US. In general there will be 3 conductors for single phase AC. Live (or Line), Neutral, and Ground. This is Earth Ground, which is not the same as common ground on DC circuits.
Wire gauge for AC cables inside the cab should be between 14 and 18 AWG. Typically 18 is cheaper and still safe so it will be that. If someone custom made a cabinet it could be whatever they had on hand.
Does your monitor require an isolation transformer? Is there one present in the cab? If there's one there I would use it regardless. Having connectors that disconnect is definitely handy, but not necessarily a requirement. The reason I assume it has an iso is those kind of terminal ends look like they could plug directly onto the secondary side of a transformer. Not sure why the ground cable would have that connector though?
If you don't do this stuff often and don't have any of the connectors on hand, a crimping tool, pins, etc... you may just want to hardwire everything together? However, real ops will look down on your work if you pass it on later. If you ever had to remove the monitor to service it in the future... having everything hard wired together makes that difficult.
I didn't know there were 28" monitors? interesting... I'm guessing that the mains voltage in UK is 220 or 240 or thereabouts? I don't know if you need an isolation transformer but if your power leads have those kind of quick disconnects on it, it would make sense to me?
It's a European 28" / 66cm. It's super common here. Like everything came in that size. I don't think you have them at all.
Anyway, Polos don't need ISOs.
More than likely the power lead he has for the monitor is actually for the cab. I wouldn't be surprised if the cab originally had a Hantarex PSU too. Hantarex used Mate 'n Lok connectors on power cables. So a power lead goes from the back of the AC socket to the PSU and then another lead from the PSU to the monitor. The connectors used for both are likely the same 3-pin ones. Otherwise the spade connectors make no sense.