Course there is an MEL for the VC10, just we call it a Go,No Go list. You cant get an Airworthiness Cert without one and as the VC10 had a civvie AW Cert then an MEL had to exist.
As Beagle reports the ELRAT is tested on Air Test and presumed servicable for subsequent flights, just as the drop down oxygen and loo flush etc is. If a crew member has reason to suspect any item of equipment is US then a risk assessment against the MEL (Go,No Go) list is made. Usually, as I remember, a captains wet finger in the air.
Or is it a wet captains finger in the air!