Coates narrowly missed out in the European 250 series
Coates' victory in Saturday's penultimate round had wrapped up victory in the 250cc Championship.
He is the final winner of the British 250cc Championship as it is being discontinued.
The Northern Ireland rider led all the way in Sunday's race.
The Padgetts team rider went into Saturday's race 23 points ahead of main rival Shane Norval.
Coates took control of the penultimate round after six laps and cruised to a comfortable win ahead of Stuart Easton and Norval.
The British success helped to make up for his disappointment in the European Championship series where he was pipped after leading for much of the season.
Coates is now hoping that the Padgetts team can secure enough sponsorship to ensure a return to the Grand Prix circuit next year.
"Last year year was such a disappointment but to come back this season so strongly has been tremendous," said Coates.
"Now the team is going to work hard over the winter to try and secure the budget so we can return to the Grand Prix circuit," he added.
Also at Donington, Paul Robinson finished second in the final British 125cc Championship race of the season to secure third place in the class.
Robinson hopes to compete in the European Championship next season.
Meanwhile, Gordon Crockard retained his British 250cc Motocross Championship thanks to an impressive series of performances in Devon over the weekend.
Two wins and a third place guaranteed the Northern Ireland man the title.
Meanwhile, North West 200 organisers have confirmed 18 May as the date for next year's event.
A big entry is expected from Britain as no major other events will take place in the UK that weekend.