I arrive in Hong Kong for a 2 day stay then join a cruise ship, 12 days later I arrive in Shanghai

You can enjoy free entry to HK if you hold UK passport, so no visa will be required.
As for Shanghai, if you will go to a third country or region rather than the last one you come from after staying in Shanghai for less than 144 hours, you may be able to enjoy the 144 hour free transit there. However, if you enter and leave by the same cruise ship, you may be required to provide a visa. You’d better confirm that with the cruise ship company.