@Theland
It would seem duplicitous to, as an atheist, attend a religion-based service of remembrance but, if it was for a family member, it would be churlish not to attend and who knows what after effects that might have, in terms with ongoing relationships with other branches of the family? Differences of opinion, over matters of faith, are not conducive to establishing new relationships with distant relatives, who you do not know well, or avoiding deterioration of relationships with the ones you do know well.
This is why I dislike seeing atheist/theist debate descending into exchanges of words, impugning another's intelligence, because this had irreverdibly altered the simple statement "I'm an atheist, actually", in casual conversation, to an unspoken but implied "I'm an atheist, actually {and you're stupid for holding any belief, contrary to mine}". Easy to do, from behind the cloak of anonymity offered by social media and no love lost, where total strangers are on the receiving end but inadvisable when it comes to the extended family. That said, once put on the spot by a direct question, from one of them, there's no evading it.
You have focused on a post-terrorism remembrance and, if I happened to be passing the site at the right moment, I would certainly drop what I was doing and join in. In practice though, it would be more likely that I would remain at home, for no better reason than I was unaware of what was planned or in the process of happening. By the time I'd seen it on the TV news, it would probably have ended before I could get there.
Joining in would, however, be merely an act of solidarity with the wider community. If none of the victims were people I knew, personnally, there would be a nagging sense of being an imposter, rather as you might, were you to attend the funeral of a (non-celebrity) person, who you never knew.
From the spirituality angle, I see remembrance as only that - remembering the person we've lost. I mourn the fact that their enjoyment of life has ceased, not the fact that I can no longer draw enjoyment from their company. What I certainly don't do is celebrate their moving on to a better place. For atheists, there is not that softening of the blow. We celebrate life and thus mourn its end, perhaps more painfully than do people of faith.