By the same train, crossing borders? Thats already a lot of countries. Not only the direct neighbours: Austria, Switzerland, France, Luxembourg, Belgium, Netherlands, Denmark, Poland, Tschechia. There are also direct trains to: Italy, Hungria, Slovenia, Suezia, Belarus, Croatia, Russia/Moscow.
It's easy to travel from the UK to Germany by train. Take Eurostar from London to Brussels in 2 hours, switch to a high-speed ICE train to Cologne taking 1h57, then take another luxurious ICE train to Munich, Berlin, Hamburg or anywhere in Germany.