ASDA Liviingston have some of the Philips Power2Go packs for £10. I managed to get the SCE4430 which is £29.34 from ASDA Direct. They also had the SCE4420 @ £10.
Probably pot luck in other stores, but worth having a look just in case to see if available in your local ASDA.
Provides up to 30 hours of portable power
This power solution provides an easy way to avoid ever running out of power with any of your portable devices on the go. When fully charged, this power solution is able to provide up to 30 hours of power without the need of a power outlet.
Connector tips are included for most brands
Due to the interchangeable connector tips that are included, this device is fully compatible with most portable devices from a very wide range of brands. Simply choose the connector tip to match your portable device.
Philips link: http://www.consumer.philips.com/consumer/en/gb/consumer/cc/_productid_SCE4430_12_GB_CONSUMER/Power2Go+SCE4430-12