I have a hybrid car and the 12V battery is under the boot floor, which
is awkward to get to if I ever need to jump start the car so I've
installed a socket in a more convenient and accessible place and I've
taken two wires directly from the battery to that socket.

The socket is keyed so there's no chance of any reverse polarity
accident occuring if I ever need to use my jump start battery pack but
I need to protect that cable run from any sort of accident resulting in
a short-circuit, so I bought a 12V circuit breaker rated at 30A (the
computers draw less than 20A to get the hybrid system to the 'READY'
state).

My question is - the circuit breaker has one connection labelled as
'Line' and the other one as 'Load', so is there anything in there that
means it will only operate with current flowing in that direction?

The wires from the battery to the socket will have a permanent 12V
supply on them. If I were to plug anything into that socket then
current would flow from battery to socket. However, it's purpose is to
do the reverse and provide 12V from an external source to the flat
battery so current flow would be from socket to battery, so will the
breaker work 'going one way' as an overcurrent device in case of a
fault but also allow current flow in the opposite direction when/if
needed?

Or should I just put a 30A in-line fuse in instead of the breaker?