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AlexS
Not sure how serious this question is, but attendance at most government press conferences (especially those at Downing Street) is usually restricted to members of the Lobby (political journalists accredited to cover political news and attend lobby briefings etc).
I can't say for certain, but doubt Piers is a member of the Lobby - although you would expect GMB's roster to include reporters who are.
Piers is not on the list - which is available here https://publications.parliament.uk/pa/cm/cmjournl/register.pdf.
EDIT: Although worth pointing out that Mark Austin isn't a member of the Lobby (based on the list), but asked a question yesterday.
Pallab Ghosh asked a question last weekend and isn't on the list either
Pretty sure Fergus Walsh has also had the BBC question on at least one occasion.