The main switch appears to be an RCCB (which is an RCD).
As you say, the 'Main Switch' appears to be the thing which the OP is calling the "external isolator", which is actually an RCD. My undressing from what the OP has told us is that the RCD at the top right is feeding all of the circuits in the bottom half of the CU. As you are perhaps implying, it could be essentially redundant, but I can't see thee external one clearly enough to ascertain its characteristics (it might not be a 30mA one).The main switch appears to be an RCCB (which is an RCD). There is another RCD top left doing something too.
Yes, now I've bothered to magnify it, you're right - 100mA, and also TD. The one in the CU (which I think is protecting the lower row of circuits) appears to be 30mA, so is not 'redundant'.100mA John
It may have been, but based on the general crapness of it all, I think complete replacement by someone who knows what they are doing may be the answer.That stuff I believe was made by Siemens.
Solution may be to put any make Rcds in a seperate enclosure, butted up to the consumer unit.