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WARNING - this information is not from a qualified lawyer. This is what we understand the restrictions are - please consult a suitably qualified person for more information
In the UK permission to launch balloons needs to come from the CAA. On application (with OS map marking launch site) and after review a permission will be sent to you and also a NOTAM will be issued. As the weather can be unpredictable it is possible to apply for a launch window instead of a single day. More information can be found here.
Hopefully these should enable you to ascertain the best areas to consider launching from when you apply for a NOTAM
Exceptions from application of provisions of the Order for certain classes of aircraft 253 (1) This article applies to: (a) any small balloon; (b) any kite weighing not more than two kg; (c) any small unmanned aircraft; and (d) any parachute including a parascending parachute.
'Small balloon' means a balloon of not more than two metres in any linear dimension at any stage of its flight, including any basket or other equipment attached to the balloon;
In the UK an Amateur Radio Licence cannot be used for downlink communications from a balloon. The particular licence section that excludes this is 9(3) which says: “The Licensee shall not establish or use the Station in any aircrft or other airborne vehicle.”\ UK Ham Radio Licence
Some Low Power, Licence exempt modules may be used - but many (but not all) of the licence exempt frequency bands specifically exclude airborne operation see Ofcom Licence Exempt sheet
There are no special rules about hamradio used airbourne. In general danish law apply up to 30KM alt.