Can I keep using my outer tube tyre with a hole in it I was riding down a nice smooth bike path the other day (at speed), and there was a branch full of thorns about 10m ahead of me, straddling the path. I couldn't have gone around it. One of the thorns pieced my tire, and pricked a hole in my inner tube - which slowly deflated. I will replace the inner tube - but is it worth me buying a new tire because of a 5mm hole from a thorn? Will this hole expose my inner tube too much? Thanks
A thorn hole in an inner tube is totally patchable.  A 50 cent patch is a lot cheaper than a new tube.  Stop consumerism!

As for the hole, it depends if the inner tube pokes out or not.  The get-home fix is to line or reinforce that area with something laid between the tube and tyre.   

On one tyre I ended up adding a patch to the inside of the tyre, which worked well enough.  It lined up against the matching patch on the tube.