As a generator on RED Plus, you supply electricity as an Exempt Supplier. This status lets you supply self-generated electricity to customers without holding a full supply licence, but only up to certain volumes:
5 MW overall to customers of any kind, per half-hourly settlement period
Of the 5 MW overall cap, no more than 2.5 MW to non domestic consumers, per half-hourly settlement period
These limits apply to your entire corporate group (including associated body corporates), not just your contractor generator.
Generation capacity is different from supply
As a generator, you may generate up to 5 MW, provided you do not supply more than 5 MW at any time.