Heimdall

I bought a new iPad Mini 3 + cellular recently, and it comes with an Apple SIM, which can be used with EE in the UK. You can buy a short term data plan without changing the SIM, or even going into an EE store. If you wish to use another carrier, e.g. Vodafone or O2, just take out the Apple SIM and replace it with the SIM from that company. The iPad air 2 also comes with an Apple SIM.

I used the cellular feature recently on a trip to Cape Town, both at London Heathrow with the Apple SIM, and in Cape Town where I had to switch to a different SIM. It worked great in both locations!