Anyhow, back to OP's query. We all find different things exasperating. As a Londoner, I personally find dismissal of valid concerns exasperating.
It is Indeed the case that, whilst it can happen anywhere, a sleepy market town or village is much less likely to be targeted for a terrorist attack than London and other big cities. (Although there is the risk anywhere of 'lone wolf' attacks eg a terrorist stabbing).
London is particularly high risk because as a pp said it's the capital. It's ridiculous to deny what the security services themselves warn about.
That said, as I told OP yesterday, millions of people do indeed go about their business in London (and other big cities) everyday with no issue.
She's most likely going to be absolutely fine, and the biggest risk in the UK is currently Covid (1000+ deaths a week).
If she does go, I hope she has a lovely time - but if she feels too anxious, she shouldn't be mocked for her perfectly understandable caution.