Can you describe a credible scenario, given the attack methods deployed in the last few years, that would be averted via hotline and "rapid" response?
If you're talking about a situation that needs to be resolved by way of a 'rapid' response, ie. within seconds or minutes, then phoning/texting a hotline is absolutely the wrong thing to do. If, for example, you observe a person behaving in a way which leads you to believe that an attack is imminent the best course of action is to alert the nearest police officer or staff member as rapidly as possible - which, in real extremis, means shouting as loud as you can - or as a last resort call 999; and then follow the "Run, Hide, Tell" advice at
www.npcc.police.uk/staysafe.
Where "See it, Say it, Sorted" is the right message, though, is when it comes to intelligence gathering. As a hypothetical example: imagine I see someone walking around a major station behaving suspiciously (such as observing staff entering secure areas, or photographing surveilance equipment, to use two extreme examples). I contact the security services and inform them of my concern. Chances are it will be nothing, but if it isn't nothing then the information I provide could be an important part of the wider intelligence picture. That is why it's so important.
It's no secret that terror attacks are becoming harder to stop as they are incresaingly low-tech and carried out by individuals/small groups. Despite claims of responsibility from some international organisations, many of these individuals have few or limited connections to established terror organisations, which makes them extremely difficult to locate. Those contacts they do have are often a) online and b) protected using encryption. One very real effect of this is to increase the reliance of security and intelligence organisations on human intelligence, including from the public, as a means detecting persons of concern.
Yes, there is a real problem with idiots and conspiricy theorists when it comes to things like the Anti-Terrorist Hotline - there's a reason it's known by some as the "Loony Line" - and part of the security services role is to filter out the relevant information from the mountain of irrelevant, if well-meaning, tip offs. But there have been several cases, and continue to be cases going through the courts, where information from the public has been vital in saving lives.
If you're genuinely interested, look out for reports of terrorism trials in the press and go and sit in the public gallery, in most cases there is no reason that you can't, although you may be excluded from particularly sensitive parts of the hearings. You'll be surprised at how many times information from an ordinary member of the public either started an investigation or forms an important piece of evidence in a case.
Of course, the chances of getting caught up in a terror attack are vanishingly small for any given individual; but the assertion by some that the chances of an attack on the rail network are small? That's simply not true. JTAC don't invent threat levels for a laugh, nor do BTP deploy armed officers because they need to find something for them to do. The risk is very, very real and - as we saw in Manchester and London last year - it remains a case of 'when' attacks happen, rather than 'if'.