Basically you are making your car look like a more desirable model and a greater risk model, so they don't want to insure you, if you make a modification you need to find out beforehand, not afterwards.
For example, I told my insurance company that I was fitting a towbar to my Peugeot, my premium went up by 70 a year but anything I was towing was then covered aswell.
You changed the agreement, so they are well within their rights to cancel the policy as you changed the conditions, not them.
As for anyone saying just don't tell them, that's very naive, I agree that for many things they will never be noticed, but if they are and your insurance is made void, you could be in for a life of financial ruin.
Just remove the rear bumper, fit the old one and finish the policy. If you have already cancelled it then see it as a lesson learnt.
As for what money they charge, i'm sure it's just the finish of that months term (If paid monthly) and then a smal cancelation fee of 25 pounds or so, it was when I cancelled a policy through Liverpool Direct, ask around though as they will try and gt every penny whether it be allowed or not.
Dave!