Grants for the Arts (aka G4A) is the main project funding scheme of Arts Council England. Open to individuals and organisations planning activities that will 'engage people in England in arts activities' or 'help artists and arts organisations in England carry out their work', G4A only supports time-limited projects — those with a clear beginning and end.
ACE's own guidance document lists some examples of the types of activities that can be funded under G4A: projects and events; commissions and productions; research and development; activities for people to take part in; audience development; marketing activities; education activities; buying assets such as equipment, instruments and vehicles; professional development and training, including travel grants over £1,000; bursaries; fellowships; organisational development to improve the long-term stability of arts organisations; residencies; touring.
Individuals can apply for grants in the £1000 - £30,000 range, but note that for applications over £10,000 the response time doubles from six weeks to twelve. Grants for organisations can run as high as £100,000 — and in some cases even higher if the application is for national activities or is discussed and agreed with ACE beforehand.
Grants for the Arts is a continuous programme and you can apply at any time. Circus doesn't have its own category/strand, so applications are generally made under either Theatre or Combined Arts. For full eligibility guidelines and details of how to apply see here.