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There are no airside hotels in LHR.
Assuming he is Japanese by nationality, he is free to enter the UK, and even use the e-gates to do so, but he must self isolate in a hotel in Heathrow or central London, and when departing he has to proceed directly to the airport. He isn't required to stay 14 days in the UK, the border is in essence open, and he is free to leave the next day or whenever. The UK and Ireland forum has plenty of advice on hotels, and the Coronavirus forum has a thread on the UK restrictions for arriving passengers. He does however need to fill in the contract tracing Passenger Locator Form, and provide his hotel name and address there. He fills this form in when under 48 hours to departure, or less, and he need either a printed or electronic copy of the form visible on his tablet or phone when he arrives in Heathrow.