if it's a straightforward renewal, and you have your old passport to send back, then you only need one of the 2 new pics signed if you're appearance has changed considerably.
don't worry about hair colour, glasses etc. passport control look at the shape of your face, ears etc to check appearance.
if signed, it needs to say I certify this is a true likeness of xxxxx(your full name) then they sign, and date the back of the pic. It's important that only 1 pic is signed.
The post office charge �6.85 to check and send passport forms...renewals should be back in 2 weeks