@SecurityWriter @Viss @mttaggart
In the UK can use licence exempt power limited options e.g. 434, 869 and 2.4GHz
I did a bit of a high altitude balloon project back in the day. Check for ideas

https://ukhas.org.uk/doku.php?id=frontpage:guides&s[]=transmitter

and @fsphil did some awesome work: https://github.com/fsphil/ssdv