As Vocky says, it's a question of pulling back the loom from the boot, unwrapping it, separating the wires you want and re-wrapping the two branches. I took the opportunity to put in new relay bases so my charge cooler relay holder would lock together with the others and also put in a new fuse holder to take the two fuses from the boot (FR1/FR2) plus the charge cooler and boot release fuses. Here's a clearer pic of the panel:

As to the prods about protecting it from water... I'm yet to be convinced that where the panel sits is prone to a dousing (and the car is now too nice to take out in the wet anyway!) but if I find it's a problem - guess what - I'll make something to cover it!
As for putting the relays in the passenger compartment, that would be trickier. You would have to extend the wiring to reach further, get the wires through an already crowded grommet and I can't quite see what the benefit would be... other than find a use for your printer!