I've read that current practice is to use several RCDs in a domestic
installation. A time delayed 100mA RCD to protect the whole installation

Only on certain types of electrical supply. Look at the FAQ.

and
30mA RCDs for sockets, outside wiring etc. My question is why can't the
main
RCD be set at a safer 30mA sensitivity since it's time delay should
prevent
it from tripping before any other ones?

There are many whole house 30mA RCDs in use and they cause a lot of trouble

Time delay 30ma is a rare beast & it's possible these days for there to be
enough cummulative (but normal) leakage in a system to make it a nuisance.
Whole installation RCDs are to protect against fire not human contact.

I thought MCBs protect against fire (overload protection) and RCDs are for
human protection


Also if you install a garage unit with a 30mA RCD on an installation
which
is protected by another 30mA RCD at the CU, which one trips if an earth
fault occurs?
Indeterminate.

And a garage should not be wired in this way.
--
Adam