They also don't flush. It's one of the most frustrating things of living in the UK as an expat. 2-3 flushes for solid waste is almost mandatory, and the issue always crops up at someone else's house.
In the capital, even homes above 2mln pounds will have questionable quality. They seem American in this sense. You really need to shell out for proper attention to detail.
1800s building materials aren't as stable as modern ones and things tend to shift over time, and indeed many old buildings are poorly built - especially by modern standards. Even the foundations don't run very deep, and London has a clay rich soil, meaning that some minor foundation movement is not uncommon in older houses. Damp is usually more of an issue, so anything wood can potentially swell a bit during a wet period, leading to things becoming slightly out of alignment over time.
Often the only way to fix stuff like that would be extensive renovations which isn't really worth it for very minor visible problems. You can tighten stuff up, rehang etc. but it will usually slowly move again over time.
Also a £2 million house in London can be just for a shell in need of renovation in some areas - you're just paying for the land and location.
Three possibilities to consider...
1. You're pressing the water saving button (piss flush)
2. You're not holding it down for long enough for it to catch - it can take even a second in some poorly maintained toilets for it to flush properly.
3. Your diet could do with some _serious_ improvement...
That being said, I'd rather having to double flush than deal with clogs as when visiting the US.
The European-style flush relies on buoyant waste getting carried away by the mass of water - which isn't the most reliable strategy.