Need help identifying shell

Hi, please forgive me as I know nothing about shells really. I got this shell at a car boot sale when I was a kid, and it’s sat in my childhood bedroom for the last 20+ years, and it never occurred to me to find out what it was, or even to confirm it’s not actually a live shell! Would anyone be able to tell from these photos what kind of shell it is, and if there is anything that can confirm it is, in fact, inert or deactivated? I’d say it’s probably about 2 feet high and 4/5 inches in diameter at the widest of the casing. Sorry I have a few pictures so there will be a fair few replies as it seems I can only post one picture per post. Thanks!

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