I'm not sure how comfortable I'd be buying a battery from a website. Partly because they're heavy, yet somewhat fragile, & really should be transported by someone who knows what they're doing, also because I'd be concerned about the supplier honouring the guarantee if the worst happened - I've had at least three batteries replaced under warranty.
If I needed one tomorrow, I'd phone round the local factors, but also I'd check the likes of Halfords & Costco - if you happen to catch a promotion they sometimes are extremely competitive. Halfords in particular have always been reasonable with me about exchanging prematurely-failing batteries; the last one had just a week's warranty left & was immediately replaced with new AND given a new 4 year guarantee - without them even testing it*. Also, as with tyres & brake fluid, I'd want to check the manufacturing date before parting with my money; no sense in buying one that's been sitting around on the shelf for too long.
* Hint: Take it back to one of the branches that doesn't have a workshop attached

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