The different brands of most fuel originates from whichever refinery is supplying the UK Oil Pipeline (UKOP) in a particular region - it's only at the distribution depots that the brand additives are added to make the fuel unique.

I doubt that Coryton will close down, it costs a lot to shut down and then restart a refinery. Petroplus, who own Coryton, has filed for Insolvency but I imagine the Receivers will be trying to sell it as a going concern. No shipments have left Coryton but it's still in production.