The problem comes if 2m distancing still needs to be maintained. For example, bear in mind it's highly likely the transport industry is still going to have a lot of staff off (the shielded groups for starters) they won't be able to run a full timetable, if people are properly spaced then realistically you're not able to get more than 10 people per carriage - even on something like a 12-car class 700 this will in practice mean only 120 people per train. Remembering the behaviour we've seen recently at parks, beaches and the likes and a probable sudden surge to go for family days out after having been locked down for several weeks, and this quite simply will cause mayhem.
Your key worker is going to be utterly furious if they board a train and find their options are to (1) stand up for the journey to work, or (2) sit within 2 metres of a family on the way to the seaside. In that situation I'd be turning round and going home, and I can say many others would likely do same.
This isn't an easy problem to square up. Of course if we could rely on the wider populace to be sensible and measured then things might work better, but so far the reaction of the general public has lurched from panic buying so key workers finished their shifts to find empty shelves, a bizarre obsession with toilet roll, a nurse spat and jeered at and told she was a "disease spreader", bedlam at beaches and public parks, and now people complaining of boredom and a sudden obsession with exercise, the latest thing being cyclists everywhere once again getting in the way of key workers. We've even seen a CMO break her own guidance and head to her second home not once but twice. None of this bodes well for people doing the right thing in the circumstances.
Generally the numbers of key workers available isn't actually that badly impaired considering how easy it is to phone in and say "my housemate has a cough", although some places have suffered worse than others. Most people are genuinely making an effort to come to work and do their bit, despite in some cases some reservations about the risk of becoming ill and seeing on TV cases of younger healthy people succumbing. In some cases there have even been some shielded people having made the decision to come to work against advice as they feel the lockdown has made it less risky and given them more control over maintaining distancing.