RSGB Contest Special Rules 2017

S7.
This contest runs concurrently with all or part of an IARU co-ordinated contest. UK&CD entrants who make all their QSOs using SSB, CW or FM will be submitted to the IARU event unless you specifically request otherwise.

On 4m and 6m MGM modes may also be used (except in CW only contests) provided the entrant meets the requirements of the IARU rules, see rule is 5.3.7 on page 60 at http://www.iaru-r1.org/index.php/downloads/Documents/VHF/VHF-managers-Handbook-7.0/.
You can find guidelines on the use of MGM here http://www.rsgbcc.org/vhf/MGMGuidelines.pdf