...
From the various NOTAMs it looks like an annual event.
The 2017 Notam -
https://www.legislation.gov.uk/uksi/...0170840_en.pdf
includes this explanation .... which leaves us not much wiser.
"As the operation does not allow the balloon, or its tether, to be lit or marked, Headquarters 11 Signal Brigade has requested that restrictions be imposed on flying in the vicinity of Topcliffe by reason of flight safety. This has been agreed with the CAA and DfT"
LFH