personally I would go bigger than 2.5 to the garage from the inside DB, you will use all sorts of power eventually, 2.5 is too small really, and go bigger than a 20 amp MCB via the RCD as well, JMO.
Better to adapt/convert the house CU so that the garage is fed from an MCB that is not RCD-protected.
(aka a "high-integrity" CU - who invented that ridiculous title?)
Locate the RCD at the garage end to replace the isolation switch. Then, if there is an earth fault, that will only trip the garage RCD and not half the house.
The SWA does not need RCD-protection, as it runs all the way from the house CU to the garage.
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