Travel expenses for self employed tennis coach

Self employed tennis coach. Coaches between 1and 3 days a week [between 1-4 hours per day] at one tennis centre and also visits other tennis clubs, as well as private houses. Can he claim the mileage for the tennis centre he attends more often or is this deemed home to office. Is there a specific threshold in which attendance is deemed 'home to office' and therefore not allowable. Seems very fluid and HMRC doesnt have specific time guidelines.