Strictly speaking from a legal point the answer is NO
If at any point the link goes outside the boundary of your property then you need an operators license.
This applies no matter how the link is provided (wire, radio, laser etc)
The thing to consider is if anyone would bother prosecuting you and the potential penalties. - The UK authorities appear more concerned about illegal radio transmitters.
I agree with picasm:
If the garage is within range then the DECT solution, using 2 handsets, looks good (you may have to experiment with the siting of the base station to get a signal to your garage).
If you avoid having a DECT charger in the garage then you couldn't be accused of operating a fixed link.
DECT handsets with intercom facility are widely available in the UK at reasonable prices. You get the bonus that, if the main base station in your house is connected to the PSTN, you could make and receive calls from your garage.
(my first post on this forum)