My EE 4G phone package for about £35 per month includes discounted roaming data for as little as 50p...£2.50 gives 100MB in all of Europe, USA, etc. Ideal for a day trip or extended trip abroad. Tether the iPad and download Notams, file flight plans etc...
PS I have a non 3G iPad and 3G iPad. Both work great in the cockpit but I pair the non 3G to external BT GPS. The 3G one I don't bother and never had a GPS signal issue.