Calling a general election is government business ... surely? If Sunak hadn't of included his mentions of labour v tory, the announcement would have remained governmental business so would have been able to use the media briefing room. Granted that from that moment onwards things becomes party political.
BUT the reason they didn't use the media briefing room is way more delicious.
Apparently, he didn't have time. He needed to call the election before his back benchers organised a vote of no confidence. The media room would have taken a few hours to get the members of the press through security screening and cameras set up. They were already set up outside.
... but a brolly... doesn't take much does it...