UPDATE: All gliders from EASA countries (including the UK, and BGA gliders) can fly without permission and restriction in Hungarian airspace! We would like however collect all papers of the gliders prior the rally to ease administration during an inspection!
According to the AIP of Hungary (source: http://ais-en.hungarocontrol.hu/aip/2017-03-02/), Gen 1.2 §3.1.2:
a. No permission to enter the airspace of Hungary is required for aircraft with a limited certificate of airworthiness or permit to fly issued by the aviation authority of EU Member States in accordance with Annex (Part 21) to Commission Regulation (EU) No 748/2012 and its amendments, validity of which is automatically recognised within the meaning of Article 11 of Regulation (EC) No 216/2008 of the European Parliament and of the Council as amended.
b. All foreign aircraft with certificate other than stated in paragraph a. must obtain permission for entering and/or flight activities within the Hungarian airspace.
The application must be submitted by the operator at least 5 days before aircraft enters Hungarian airspace with the purpose of the flight, intended destination aerodrome (if any), expected route through the Hungarian airspace to the NTA AA.
The application for permission shall be substantiated by copies of the following documents:
1. Certificate of Registration
2. Airworthiness Certificate including established operational limitations (if issued)
3. Permit to Fly including the conditions (if issued)
4. Proof of annual inspection (if issued)
5. Differences from the ICAO Annex 6 and 8
6. Noise Certificate (if issued)
7. Proof of third-party liability insurance
8. Air Operator Certificate or equivalent if any
9. Congruence RVSM, B-RNAV, Mode-S, ATC, ACAS II, FM immunity
10. Licence of pilot-in-command
The permission may be issued with time and/or any other operational restrictions. Duration of the permission will generally be granted for the purpose by the end of the validity of the Permit to Fly.