If you have an Apple or Android phone for tablet, then initially you should try the
totally free UK AirspaceAVOID.
It's totally free, airspace and aerodata is constantly updated and the app[ is NATS approved.
However it doesn't do flight planning...it just warns you if you are about to enter airspace.
If you want to do something more on your phone (or better iPad or Tablet) try
EasyVFR by PocketFMS. It costs €150 per year for an all in licence (maps, NOTAMs, advanced weather forecasts + METAR & TAF) and unlimited free flight plan filing which you can use on all of your devices (Android, iOS, PC). Or you can purchase a licence that is restricted to one operating system for €69.99.
If you are sure that you want to choose just one of the two mentioned in the begining, I'd go with a Garmin device.
dp