There is no electric train from London to Hong Kong.
The only trains between London and Hong Kong are electric. Your train needs to be electric to enter both London and Hong Kong. The Trans-Siberian Railroad only uses electric trains.
The Channel is crossed by ferry, which uses fuel.
The Channel can be crossed by ferry, but if you are coming out of London, you are probably taking the Chunnel. The Chunnel, like most long tunnels, requires the use of an electric train.
Unfortunately, the cost of energy in Europe right now is making this journey more and more impractical.
Train travel is the most efficient land transportation over long distances, so it is becoming more practical as prices rise. Most of the trip is in Russia, so prices are even cheaper than you would think.
