Thanks both. I should have clarified that I do have an HDR compatible box and can watch UHD sports such as F1 or football in HDR without a problem.
It transpires that it's just Sky putting things in stupid places. Looking in one of the other sections under Sky Cinema reveals plenty of titled in UHD HDR, yet everything in the dedicated "Ultra HD" section lacks HDR.
This stupidity even extends to the same title. For example, if you locate Top Gun: Maverick under the "Ultra HD" section, it's only in UHD without HDR. Find the same film under the "New to Sky Cinema" section and it has HDR.
Hopeless.