Hi
I came on to ask the same question about GP letters, to be honest. I had heard you could get a registered card, but cannot find any evidence about this, so shall be hot-footing it down to the GPs too.
I have travelled with diabetes for a significant number of years now and have never experienced highs or lows on aircraft, but the one thing I have learned is that we all react in different ways to different experiences, and so to act with caution and document your own path for future reference.
However, in my personal experience, I happen to metabolise insulin harder and faster in hot climates, so am far more to prone to hypos, and have to significantly reduce my dosages in hot countries. This may (OR MAY NOT!!) happen to you, just be aware of it as a possibility if you're heading to anywhere particularly hot so it doesn't take you by surprise.
Hope this is of some assistance