The image you added to your post does not show a UK hallmark.  As a minimum, it should have the maker's mark (initials), the metal purity, and the Assay Office mark.

All jewellery listed on Ebay should follow the jewellery policy.  This becomes very important when jewellery has to be authenticated.

This is the key part - the link to the 'Guidance Notes' will show you what a full hallmark looks like.

• Items must meet the requirements of the Hallmarking Act 1973 - opens in new window or tab. For more information on hallmarking, please see t...Hallmarking Guidance Notes from the Assay Offices of Great Britain - opens in new window or tab
• To ensure that you meet any obligations under the Hallmarking Act 1973, where applicable, you should add this notice on hallmarks - opens in new window or tab as an image to your listing