If you have a Windoze device, then you have various options.
Memory Map will run the CAA charts, which (1:500k) are about £25 each and for the whole UK you need three of them.
Oziexplorer will run any map whatsoever which is available in one of the common graphical formats, and which can be "calibrated". Maps which have a lat/long grid are trivial to calibrate (all aviation charts); ones which don't (e.g. city maps) are more tricky (people tend to use google maps to locate some landmark coordinates; not accurate but better than nothing). There is a vast choice of free maps for Oziexplorer, on P2P / underground sites, including the usual aviation charts.
Then you need to connect a GPS to it - bluetooth is a popular method, which allows the GPS receiver (£20 on Ebay) to be located where it can see the sky properly.
However, a laptop is not good for airborne use; a tablet format is much better. This is another long story but basically the challenge is to find one which is sunlight readable.