The CT scanner is usually owned by the NHS trust, which makes the marginal cost of using it rather small. It's almost the other way round: having bought a big capital asset, the incentive is to make sure it's kept busy being used effectively. The NHS "internal market" obfuscates this hugely, but there is still no incentive to over-provision because the system can't drive up external insurance costs.

See e.g. https://books.google.co.uk/books?id=RyJfEbwGwgkC&pg=PA4&lpg=...

(random googling found confirmation of this in someone's FOI request: www.heartofengland.nhs.uk/wp-content/uploads/FOI4797.docx )