I Have to Speak More Slowly

The problem your friend might be having in London with understanding different people could be because they're not all native speakers. So for example, it might be an Indian person who's speaking English with an Indian accent or a Chinese person speaking English with a Chinese accent. The accent in London can be difficult to understand if you're not used to hearing it. Don't just limit yourself to standard British English - only 2% of the world speak that way.