If you are not flying from Nice to London and want an alternative mode of transportation, trains from Nice to London are a great option.
There are no direct rail services between theses cities, so to take a train from Nice to London, you should first take a high-speed TGV train or overnight train from Nice to Paris. You can find more information about this route here.
Once in Paris, you can take the Eurostar train from Paris to London. It is currently the only rail service between the French capital and the British capital. You can find more information about this route here.
Keep in mind that most trains from Nice to Paris arrive at Paris Gare de Lyon and Eurostar trains from Paris to London depart from Paris Gare du Nord.
As such, be sure to leave yourself enough time to make your connection in Paris, either by taking the direct RER D train to get from the Gare de Lyon train station to the Gare du Nord train station, or by taking a taxi or bus.
Also remember that the Eurostar requires you to check in at least 30 minutes prior to departure, so plan extra time to account for that.