I have a raspberry pi hooked to a small 11" display and use 'Viking maps' with an external usb GPS sensor (mouse?) - the whole thing needs very little electric. There's also a couple of other bits of software running on there, mostly to support and extend Viking itself.
Some assembly required, batteries may not be included etc... But it works well and had been very reliable. Added benefit that it can directly import Google's kml files, making trip planning and management in a shared group much much simpler.
Other than this, Ordinance survey have an app that allows for subscription based map access, so travelling in the UK, just subscribe for the months you need.