In addition to the halfords kit, get some ring spanners. 8 - 20mm for starters. Also a set of maul grips, long nose pliers, a small flat bladed screwdriver, torque wrench, breaker bar, a cheap little multimeter, a wire brush, and a code reader. You could go on and on but I think these are pretty much essential in addition to that halfords kit to get you started. Can get everything for under a oner if your lucky.

Oh yeah and obviously trolley jack and axle stands as well! So maybe £150