It's intimated it will commence from 1 June 2020 and reviewed every couple of weeks and currently expected to last until at least mid July.
The "quarantine" will consist of self-isolation but you can use public transport when you arrive in the UK

They also indicate you can get fined up to £1,000 for breaking the quarantine and there will be spot checks.
"Any passengers arriving in the UK by plane, ferry or train would need to provide UK Border Force officials with an address where they will self-isolate, otherwise accommodation will be arranged by the government."
And contrary to their initial mention of this situation a few weeks ago - there is no exemption for arrivals coming in via France.
I think they quickly realised everyone was researching how to get back home from their holiday the quickest and cheapest way by that route....