CIV is a set of rules shared by European rail operators to protect international journeys by train. You can add it to UK train tickets by selecting a CIV ticket from search results.
These special fares, that are are only valid with proof of an onward Eurostar ticket, mean that the domestic UK leg of your journey will be protected under international conditions of carriage.
This means that if your UK train is delayed you'll be permitted to travel on the next available Eurostar. The tickets also include travel on the London Underground (if necessary) to/from London St Pancras International.
CIV fares are priced differently to normal tickets, and are only valid with proof of an onward Eurostar ticket.
CIV protection is not a substitute for travel insurance.
Prices are usually slightly more expensive than non-CIV tickets for the same route, though they can sometimes be cheaper, especially during peak times.