An overrated time in British cinema, imo- many have dated badly and some "swinging sixties" efforts come across as very sub-par French new wave wannabes or striving too hard and theatrically for grim realism.
As well as Kes and If, try The Servant (Losey). Billy Liar is also worthwhile, hardly a masterpiece, but with the great advantage of Julie Christie who epitomises the era. Loneliness of the Long Distance Runner (also with Tom Courtenay as main young protagonist) perhaps has the edge. Antonioni's Blow Up not up to his highest standards but very typical of the period.
Alfie was recently voted by Empire readers the best of Bristh that decade, but it's pretty naff. The Italian Job was a flag-waving crowd-pleaser from 69 also with recent Hollywood remake.
Not forgetting the Bond films and Pink Panther (not dramas admittedly)....