Strictly speaking, you should tell them about every little accident. This is true even if there was no damage caused, even if you paid for it yourself, even if another driver admitted liability and paid for any damage to your car. The reason is: the insurer is concerned with the likelihood of you having an accident in the future. If you have had a minor collision but paid for the damage yourself, then (in the eyes of the insurers) you are still more likely to have more mishaps. What they don't know about, they can't worry about, but it is usually a condition of every policy that you are completely upfront about every crash, minor or otherwise, that you have been involved in. So, again, you should let them know. Will you? It's up to you.
Most vehicle insurers and brokers have special offers from time to time, and sometimes these are quite generous. The problem is that there is no way of telling when they will be available. This is a good reason to use a price comparison engine, because it will get you quotes from a very large number of insurers which should include their latest offers.
However, how impartial are they? Remember that most larger comparison sites are owned by insurers who have their own products to promote. Are their results really impartial? Do they necessarily show the cheapest price available? Remember that none of them compare policies from every single insurer in Britain, and many (most) only display offers from those that pay them a commission. Someone, after all, has to pay for those lavish TV adverts. So, sometimes you will get a better price from a small broker (if you can find one) particularly if you have had a number of accidents or convictions, you have medical issues to take into account, you drive a rare or valuable car, or you differ from the normal motorist in any other way.