Charlie,
If your brakes are not in top tip shape then there is a very good chance that your track session will be prematurely cut short by either brake pedal fade or pad fade which are two completely different things.
For the sake of £300.00 why take the risk in spoiling your trackday?
There is nothing worse than getting in a few laps and your brake pedal sinks to the floor.
When selling your car, new brakes will also be a good selling point, however you will not get any more money for your car but it will make it easier to sell with now pads, lines and fluids fitted.
In answer to your specific question Charlie, I would advise you to definately change the lines as well, and at £72.00 it is a no brainer.
Alex.