Windows app development
It doesn't have anywhere the amount of users as iOS and Android, but in certain situation, you may require a Windows app, or have an existing app you need help with.
What kind of market share does Windows have?
iOS and Android account for the majority of mobile phones operating systems in the UK. The percentage of people who use Windows phones is much lower than these two dominating forces - of all of those with phones, the percentage of those using a Windows phone doesn't reach double figures. But that small percentage of a whole could account for your target audience.