The official UK Ship Register site seems to omit this important information. However, the letter issued with the SSR states the following
You must ensure that your vessel is marked with its registered number (including the prefix “SSR”). This must be painted on, or fixed to the exterior of the hull, deck or superstructure where it can be readily seen.
Each digit of the number should be a minimum of 30mm in height and 20mm in width, except for the number 1, which should be 5mm in width and have a thickness of 5mm with a space of 5mm between each digit.
Any plate or raised lettering used to display the number must be securely attached to the vessel and should be effectively mainitained and renewed where necessary.
Regarding Port of Registration (note: not necessarily the same as Home Port) this is not mandatory (AFAIK), but it is customary to include it under the boat name on the transom.