You will need RCD protection somewhere is virtually every case. If it is fed via SWA cable so no need for protection on the distribution cable then it makes sense to have just a MCB at the house CU and then RCD and any other MCBs at the garage.
However, if you can't be sure of the sub-main installation then having RCD/RCBO at house and then just MCB at the garage is a better choice.
Two 30mA RCD in series without selectivity is poor design, not enough to worry about on an EICR beyond C3, but not something you should aim to do unless you have a very good reason.
Having said that, having MCB in series or RCBO+MCB will have negligible fault selectivity, even if they are OK on overload due to a 1.6:1 or greater difference in rated current. Again, not good design, but usually an acceptable trade-off if the final circuits in the garage are not critical as done in practically every case.