I'm reminded of a cautionary tale from last year. An applicant had a conditional offer from Bristol and a higher conditional offer, by one grade, from Imperial. He went firm with Imperial and insurance with Bristol. Come August, he unfortunately missed the conditions of Imperial's offer by two grades and hence missed Bristol's by one grade. He was subsequently rejected by both universities and was left without a place at all, despite having pretty good grades in general. I gather he did find a place elsewhere but only after a very stressful experience of phone calls and clearing.
If you miss your firm choice conditions, it's their discretion whether or not to take you anyway. Lots of people still get in as "near miss" acceptances. We aim to work this way, as it's really the only way we have to control our actual numbers. If your firm choice doesn't take you as a near miss, but you've met the conditions of your insurance offer, then your insurance university is required to admit you.
If you put Bristol as insurance and you then miss our offer conditions, we are extremely unlikely to admit you. This is because we always work through the "firm" near miss candidates first. That's because we can be confident they'll accept the place. An insurance near miss could still be taken up by their first choice. We normally have enough firm near misses to fill our places, so insurance near misses rarely ever get a look in.
So, an insurance choice just one grade lower than your firm choice is pretty risky, or even no insurance at all. Miss by one, and you still stand a good chance of getting into your firm choice. Miss by two, and you could get nothing.
Bristol could be the right insurance choice for you. Maybe you somehow know you won't miss your firm by more than one grade. Or maybe you're committed to withdrawing and resitting If you don't get Bristol's grades or higher. Those seem unlikely, but they are consistent with how insurance works.
BUT if Bristol is your second choice, then that doesn't make it a good insurance choice. Or perhaps you can't decide between the last two but you let the grades do it for you (as there's no point accepting a harder offer as insurance than firm). Your insurance choice ought to be the lowest offer you would be prepared to accept, and that's not necessarily the same as your second choice.
This post isn't intended to just pile extra pressure on making your firm choice. I'm aware this might sound a bit like "firm us or nothing" and there's a lot of that around. But please just think it through - I don't want more people to end up like my cautionary tale simply because they didn't know the risks.