Definitely longer than 20 minutes! My son said 40 minutes for the commute. They used to go to a different pub once a week with one or more of the Arcadia professors - learned a bit of London history and really had fun. There are also plenty of excursions to join. Recommend going on the very early half-day adventures. Reasonably priced and they quickly feel comfortable exploring on their own.
@mominsb thanks for the feedback on my commute estimate. I did say about 40 minutes door to door (20 minutes in the tube itself - which I checked against the London Underground app - which said 18 minutes so I rounded up!). If your son’s commute experience was different then that’s good to know.