If you only bought airtime credit top-up and not a goodybag, then using your phone can work out quite expensive, especially data, at 5p per megabyte - if you're streaming music, then depending on the setings, your £10 could be only about 200 songs as sound only, or maybe 20 music videos.
Even if you're not using data yourself, many smartphones have loads of apps running in the background, using data all the time for things like downloading updates or checking for new messages. It's always best to keep mobile data turned off until you actually need it. Also, find the settings to do app updates only over Wi-Fi, and turn off background data on as many apps as possible. If you have an iPhone, turn off Wi-Fi assist,, which uses mobile data at the same time as Wi-Fi to make it faster, or if you have a recent Android, see it it has "Smart Data", and turn that off- it's basically the same thing.