Presumably the electrics will be SWA as it's going underground and to the garage?
If you can put some conduit in for the 5m underground section, use it just for the Cat6 and then just just buy standard external rated Cat6 cable (like this stuff for £21) and direct clip the rest along the walls etc... But you should keep it a good few (ideally 6-8") apart from the SWA, although interference is unlikely with SWA it's best practise (might even be in regulations somewhere) to keep it a suitable distance apart as suggested.
Or if you don't want to run conduit for the underground bit, you could buy 30m armoured external Cat6 cable (here for £68). Just means it'll be more of a pain to run the cable (less flexible) and terminate (removing the protective steel wire etc). However it would be a proper job and you should have no worries that trees/roots/rats could eat into the cable, or someone accidently digging into it there would be much better protection.
Still keep it a few inches apart from the electrical run regardless, but I'd definitely get the Cat6 done if you're paying labour to have the electrics being run from/to the same area!
With cable of that thickness is there any particular connectors you'd reccomend? In the past I've had trouble with this.