Yes. There are (or used to be) to-go containers. Lots of people stayed late enough to just grab food, when I was there. You already stayed that late, it's fine. You can go home without judgement.

Further, a not-insignificant fraction of people stop working in the 4-6 range, do non-work activities on campus (gym, volleyball, various classes), and then eat dinner and leave. I've certainly never felt any form of pressure to work until (or past) dinner.

When I was there (London, finished 2012) they stopped these - apparently some folk were loading up and feeding their families.