The withdrawal of the British will result in less money being available. Either the EU must reduce the budget or net contributors must pay more. However, the EU partners have different priorities and the research and innovation budget must be adopted unanimously. While the wealthier net contributors such as Germany, Austria, Sweden, the Netherlands, and Denmark want to cut their total expenditure to one percent of gross national income, the net recipient countries are less concerned with research and innovation from which they would not benefit to the desired extent. An increase in the research budget compared to the old Horizon 2020 would only be possible at the expense of agricultural subsidies and the Cohesion Fund for poorer Eastern European Member States. And the countries involved do not agree with that.
The Commission has now called the Member States to take a decision in the near future.
Source and further information:
https://sciencebusiness.net/framework-programmes/news/new-research-commissioner-faces-fight-eu941b-horizon-europe-budget