Check with your specific airline. I don't see anything on the TSA list, don't even know where to search for the rules in the UK, but American Airlines specifically lists "lead-acid batteries" in their list of "All items listed as Prohibited Articles on aircraft: "

TSA does list "Spillable Batteries" as prohibited - you'd have to judge whether your battery falls in that classification.

There are plenty of Google hits that I would be concerned if I were you.