The UK calendar is not meaningfully different to attending many quarter-based universities in the US, where you finish in mid-June. Oxford’s end of term this year is Sat Jun 17, Stanford’s is Fri Jun 16.
Quarter-based students have some internship complications, but generally employers and think tanks who hire students from places like Stanford will have a second start date in mid to late June. My S did a think tank internship in DC that started on June 20th.