The cable can be ran internally and would be preferable in that set up. In fact as it is entirely internal, cheaper cable should be able to be used as it doesn't need to be armoured. There may be a reason why they can't come off the main consumer unit, you could all them and ask.
A lot of installers like to just split the meter tails and install a new mini consumer unit so they don't have to touch the existing installation, based on the quote, I think that is there plan for you.
Are you planning on parking your cars inside the garage or do you just want the equipment out of sight?
I have mine on the middle pillar of my double garage but its on the outside. We never park cars in the garage so opening it up to plug in each time would be hassle I couldn't be doing with.
For reference my Hypervolt install was £880 but it was on a detached garage. It was about a 20m run (about 1.5m underground) and the installer split the existing meter tails and installed a new mini consumer unit under my meter cabinet which is down the side of my house.