Luckily we aren’t in the Arctic and London will mostly avoid extreme cold weather for long periods of time. Therefore it is normally fine to lay concrete in cold weather in London as long as you follow the correct procedures and protect the concrete as much as possible.
Freshly laid concrete must be protected from freezing for at least the first 24 hours. Because water is a main component of any concrete mix it is very important that concrete being laid in cold weather is protected as much as possible from freezing after it is poured. If ice forms in your fresh concrete mix it is likely that the expanding water crystals will disrupt the cement’s hydration process, and can cause the concrete to lose up to 50% of its designed strength.