Are you coming out of your household consumer unit? Ideally, you need to come off your consumer unit and go direct to a separate consumer unit with RCD for the garage. You want the cable under you garden to be SWA (Armoured cable) but also protected incase of any damage etc, hence coming out of your household consumer unit..... 6mm2 SWA will be ideal and will cost you about £125inc VAT for a 50m roll.

All qualified electricians feel free to add!! I'm still at college learning the trade!!