1) Could I build the garage around the meter? I'm guessing probably not.

This might actually be possible, although they do prefer them outdoors so
they can read them easier.

2) Would it be a very expensive job to get the meter moved round to the
front/back of the house?

Probably.

3) I imagine having the flue rerouted wouldn't be a major headache. Is
this the case?

Depends on the boiler and availability of parts. What is the make and model?
Is it open flued, balanced or fanned?

Christian.