Re: I understand why but it's a bit of a bullshit charge
While owning those is absolutely legal
I'd advise a little caution. You're reliant on a court agreeing you have a 'reasonable excuse' for possessing them. Your membership of an organisation doesn't in any way affect the fact of them being likely to be useful to terrorists. They either are or they aren't.
While there's a fair amount of precedent at this point, juries tend to be generous with the excuse definition, and the CPS doesn't waste resources against people they don't actually want in prison (perhaps for some other reason), the burden of proof is still on you to explain your possession. That's a bit different from something being absolutely legal. Waterstones' employees have the very reasonable excuse that they're selling the books to buyers who have a responsibility to ensure they in turn have a reasonable excuse. Hopefully you do have a fair number of other military history books, records of visiting exhibitions or association with other enthusiasts and so on to back up your academic interest.
Incidentally if this N.Ireland arrest is related to the spreadsheet leak it's likely to be one of the slightly different (and arguably less bullshit) offences which specifically criminalises information about police officers. For example section 58A presumably makes it an offence to even ask someone to give you this spreadsheet:
"A person commits an offence who—
(a)elicits or attempts to elicit information about an individual who is or has been—
(i)a member of Her Majesty's forces,
(ii)a member of any of the intelligence services, or
which is of a kind likely to be useful to a person committing or preparing an act of terrorism, or
(b)publishes or communicates any such information."
(It would also criminalise the original leak if it were intentional, but that doesn't appear to the case)