Silver gives you the basic travel cover you should need for your trip. Gold adds things like missed connection cover, and provides increased cover for medical expenses and cancelling your trip, while Diamond gives the lowest policy excess (the bit you pay towards a claim) as well as the highest cover across all aspects of your policy.
A single trip travel insurance policy starts from only £8.44. Price based on a healthy individual aged 34, travelling to Europe for 3 days on a Silver policy bought online. Travel Insurance data, 18th April 2018.