Only problem is it has a standard non-rechargeable lithium cell and the solar panel isn't actually connected!
I've got some solar cells from this german ebay store. With this one I have powered an Atmega and a LCD. But it works only outside. A Lamp or sun shining through a window doesn't provide enough power. The 5V / 100mA of the cell is only reached with direct solar irradiation.
I think the modules are from china and should be available in the UK, too.
Btw: My espruino has arrived today (January 29) in Germany