If you're talking about main LV switchboards fed direct from Distribution TX's then you are in a whole new ball game i'm afraid, where you will need to know your onions along with how to setup and test protection relays. The bigger and more complicated the Switchboard (eg Switchboards connecting several TX's with bus couplers galore and Stand-by generators thrown in for good measure) the further out of your depth you're going to be. Compared with testing and commissioning LV/MV Switchboards, a BS7671 EICR is a proverbial walk in the park!!
You'd need to be conversant with the various protection relays, how they are integrated into the system and work. You'd also need to be conversant with both primary and secondary injection test equipment, and those bits of kit don't come cheap at all. In fact, to cover the cost of the test equipment alone you'll probably be looking at the very least £12K and probably more, for secondhand or cheap budget test equipment!!
To give you an idea, take a look at our sponsors site. then look up ''Primary Injection'', ''Secondary Injection'' ''Ductor testers'' and depending on how deep you intend offering testing services, a good deal more on top!!
As to where you would gain training, pass i don't know. Most of the guy's we have used over the years have come from backgrounds in switchboard manufacturer, and have quite an extensive knowledge of all the protective relay manufacturers products etc. This field of work is so diverse and varied, it's hard to put any hard and fast minimum training and degrees of experience required.
This is a truly specialised area. Get into it, and make a name for yourself, ...and you can make some very serious money!! As you will be in demand from every major and not so major project around the world!! lol!!