Source? As someone that comes back to London every month, I’ve been able to roam the same as anywhere else in the UK. I’d be shocked if this were true.

Did the UK stop people from just picking up a cheap SIM at the Airport? I always like a local number when traveling. Anyway, Indian Roaming plans are so cheap these days that it's much easier and cheaper to just subscribe to them as part of the plan. These days, I don’t even need to add/activate it or anything, the providers turn it ON when I start my phone outside India and turn it off when I re-activate back in India.