It's a decent enough price, but surely you're going to be better buying one locally than online? Hardly anyone will transport batteries so delivery wouldn't be cheap.
Halfords prices are reasonable; I bought a new battery for the A40 recently and that was £40 for an 062 - they range from that up to about £80 (you can get a baby 038 for £30 but they struggle to hold charge for long even on Minis).
You might do better from a local battery specialist or motor factor, but Halfords were cheaper than my local parts shop.