There are two good fast & comfortable options for travel by train from the UK to Serbia. Option 1 involves a morning Eurostar to Paris, a high-speed TGV to Munich, sleeper train to Budapest and EuroCity train to Belgrade. You get a morning in Budapest into the bargain! By all means stop off for longer on the way, each train is ticketed separately so it costs no more to spend an extra day in Paris or Budapest en route. Option 2 is simpler route, involving just 3 trains and so easier to book, using an afternoon Eurostar to Paris, sleeper train to Munich, and direct EuroCity train to Belgrade, arriving late evening the day after leaving London. This can work out more expensive than option 1, but there's a way round this, explained below. Finally, option 3 is to go via Vienna, using the Vienna-Belgrade overnight sleeper which gives a morning arrival in Belgrade.