It would be slower, yes, but you could still get from London to Beijing in a few days, which seems reasonable when you look at historical travel patterns and the sheer inefficiency of the alternatives.
This is not 1290 when Marco Polo was traveling around. This is 2022 when people have seen much faster and nicer alternatives. Nobody is going to put up with days of travel when they can fly anymore.
I'm not convinced trains are more efficient than planes at that distance either - I don't' know how to analyses it, but planes see much less wind resistance because of altitude.
I agree we should do more trains, but they are limited to around 1500km per trip before flying is enough better that you look like a fool for suggesting it.