It might seem surprising that the activity of the sun could affect mobile communications. But occasionally large solar storms create an increase in radio noise, which could reduce the sensitivity of base stations. They also disrupt the ionosphere, which upsets the availability and accuracy of GPS (which helpfully is not much used in UK mobile systems, but there are exceptions).
We are assisting on a committee of the Royal Academy of Engineering which is considering these issues and will issue a report to provide advice to government and other bodies on these issues. See this call for evidence http://www.raeng.org.uk/news/releases/shownews.htm?NewsID=729 for more details.
The extreme storms we are considering might only occur once in 200 years: but the next time could be tomorrow, and we want to ensure that the UK has considered the possibilities and has ensured that any critical systems are appropriately robust.