Herne Hill Bike needs

Also, i'm currently running 42/16, so would I be better going for a 45/13 setup, or a 42/12 but having to shorten and lengthen the chain when I arrive and when I leave, as I will be cycling from Uxbridge.

always a bit of argy bargy with your ratio if you have a small chainring...

long term it's better to chose a ring that can be used on the road and track, then flip to a smaller cog when you get to the track. not always possible to use the same chain, depends how long your track ends are. I have two chains now, one for 50/15 and one for 50/19.

12t sprockets are rare and not a great idea.