This has been an interesting post to read. I would like to add that generally it is an individual MCB that is most likely to 'pop' and then require resetting. RCD's, RCBO's, RCCB's are less susceptible to tripping in a typical domestic set-up than good ole MCB's, unless you have a dodgy ole fridge and 30 PC's in there!
I understand the OP's point about discrimination; ideally a more sensitive RCD would be utilised in the garage, this would hopefully minimise inconvenient tripping in the house. You are possibly more likely to get nuisance earth faults in a damper outbuilding (??!). I would 'Henley' block the tails and have a front end RCD feeding the garage via SWA (if necessary). Effectively turning the dual board to a three-way split board. Or a 5/5/3 (High integrity board would do it).
I understand the OP's point about discrimination; ideally a more sensitive RCD would be utilised in the garage, this would hopefully minimise inconvenient tripping in the house. You are possibly more likely to get nuisance earth faults in a damper outbuilding (??!). I would 'Henley' block the tails and have a front end RCD feeding the garage via SWA (if necessary). Effectively turning the dual board to a three-way split board. Or a 5/5/3 (High integrity board would do it).