Ahh yes, did not spot the significance of crabtree starbreaker. And you can see the plastic pillar too.
Well, the wylex MCB wont have the tag on the back so perhaps it will pass in front of the bus and not connect (not tried this, looks tight), and can have a bit of 4mm or whatever to pick up a live from the incomer like the RCD s do.
or it can bunk down in one of the RCD terminals.
I may have preferred an RCBO as I presume there is no RCD on that cct
I suspect the wylex breaker on the 'non rcd' side is simply looped into the outgoing line connection of the main switch. if they're good, they'll have used a bit of busbar.