As you probably know, the UK Government and the European Commission confirmed earlier in the summer that the UK EU referendum result had no immediate effect on UK organisations applying to or participating in Horizon 2020 and that they could continue to apply to the programme in the usual way.
Statement from Jo Johnson, MP: https://www.gov.uk/government/news/statement-on-higher-education-and-research-following-the-eu-referendum
Statement from European Commission: https://ec.europa.eu/programmes/horizon2020/en/news/outcome-referendum-united-kingdom
In addition to these statements, we are pleased to inform you that the HM Treasury (the UK finance ministry) has guaranteed that all UK organisations bidding individually or as members of consortia for Horizon 2020 projects, while the UK is still a member of the EU, will have certainty over future funding with the Treasury underwriting the payments of such awards, even when specific projects continue beyond the UK’s departure from the EU.
Statement from Chancellor Philip Hammond: https://www.gov.uk/government/news/chancellor-philip-hammond-guarantees-eu-funding-beyond-date-uk-leaves-the-eu