Here is my understanding, from a layman's perspective.

CT Scan (particularly with contrast) is great for showing structures -- i.e. bony or soft tissue growths -- tumors, lymph nodes, etc. However, by itself, it cannot definitively determine whether a growth is cancerous -- there are certain characteristics that can provide a clue on CT, but nothing diagnostic.

MRI is basically the same in terms of function, though through a different mechanism. Some doctors prefer MRI for clarity in soft tissues, but CT with contrast is close. Again, however, there is nothing definitive in terms of cancer diagnosis.

PET (Positron Emission Tomography) actually visualizes cell activity. You are injected with a radioactive isotope that binds to glucose, which cells use for food. Since cancer cells are "hyperactive", they consume glucose at a much higher rate. This shows up as a "hot spot" on the PET scan. Unfortunately, many types of inflammatory processes also show as areas of increased activity on PET scans, as the immune system cells also consume glucose at a higher rate when battling infection or inflammation.

Thus, most CCCs in the U.S. use a combination of the CT and the PET. If an area of increased uptake coincides with a mass seen on CT, it is highly likely to be malignant. PET is fairly sensitive as well, catching very small cancers.

In terms of the difference between U.K. and U.S., I can only hazard a guess, based upon my travels in the U.K. and Canada. Scanning technology, as a rule, is less prevalent under the nationalized health model than it is here. Friends from Canada routinely come here to Ohio or to Michigan to receive MRI, CT and PET scans, due to the difficulty getting scans in Canada. I have heard the same thing from friends in England.

PET is a newer technology than CT, and even here, the number of machines is much smaller than CT or MRI. It is likely the supply of PET scanners - and the ability to rely on the CT scan + biopsy to achieve the same result.