I did Waterloo-Cologne-Waterloo many years ago (before any of the high-speed lines except Calais-Brussels opened) One change in Brussels, a five-hour trip, very easy.

The biggest problem was that nobody knew how to sell me a ticket for it. I think Eurostar have their act together rather better now, but it was a pig and a half just buying the ticket.

My colleagues flew and thought I was nuts to take the train -- my reasoning was that if I would normally take the train from London to Glasgow (also five hours), why not London-Cologne?