Why was this film made in London.....

I'd imagine that it was made in London because a lot of it was shot on location and most of the locations were in the vacinity of London. Plus London is pretty much the centre of the film industry in Britain so it would be easier to get costumes, lighting, set design there as opposed to it being shot in Scotland.

Actually, the majority of the movie is filmed in the Czech Republic.

Casting is all about who fits the director's interpretation rather than where the shooting takes place.

Probably the same reason so many American films are filmed in Canada, or indeed places other than the USA.🐭