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.
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