Went for a two hour circular flight this afternoon from Heathfield (East Sussex) to Rochester, along to Margate, touch-and-go at Manston and then back to Heathfield via Folkestone.
Had the dongle (on Vodafone) just loose in the cockpit for the first leg. For comparison we had a mobile phone (on O2, as that's all we had) in the cockpit. We noticed that the dongle had coverage almost all of the time as did the phone. They both dropped out briefly at similar points.
After stopping at Rochester I plugged in the external aerial and hung it in the cockpit. It didn't seem to make much difference but again we had connectivity almost the entire time with a couple of drop-outs.
I wonder if positioning the dongle / aerial at the bottom of the aircraft - so no aluminium in the way might make it even better. But for me - the fact that it was at least as good as the mobile phone - it's a success. And I shall now purchase a dedicated SIM. I did a speed test at one point when the dongle was showing 1 out of 4 bars of 4G signal : ping 49mS, download 1.6Mbps, upload 23Mbps.
And the £45 Amazon fire tablet worked perfectly (although it's possibly not as bright as other tablets)
I'll update the document to include the details of the external aerial at some point.