Microsoft's Universal Foldable Keyboard is currently on sale at Amazon in the UK. The online retailer currently lists the accessory for £47.99, saving you just over 50% on the original price. If you're looking for a portable keyboard that can hold a decent charge, you'll definitely want to consider this solution from Microsoft for your mobile setup.
So what does the £47 bag you? You'll be able to enjoy a keyboard that's 5mm thin, supports Bluetooth 4.0, can last up to 3 months on a single charge, is resistant to spillage, works cross-platform with Android, Windows and iOS, and has a magnetic closure to ensure it remains closed when you're carrying it around.