If you innocently open a spam e-mail with dodgy material on, then you have nothing to worry about - think of it as driving at 32 m.p.h. in a 30-zone. The police will be looking at you if you did 85 twice a day for a week - that's not an innocent mistake, that's flouting the law.
The police involved in tracking do have information from ISP's and appropriate monitoring agencies, but they are interested in the serial observers, and the sharers, rather than the one-time curious, and certainly not the inadvertent viewers such as yourself.
Sadly, child porn is on the increase, and police resources are so limited that they have to concentrate on the serious sharers and distributors - as in Operation Ore.
Remember, there is a world of difference between clicking inadvertently on a site, and the straight off again, or opening a spam without knowing the contents - as against deciding to take up a subscription offer which involves giving credit card details, that is a conscious decision to perpetuate the abuse of children, and deserves anything the law can provide to punish and prevent it.
Hope this sets your mind at rest.