More than 125 firefighters are at The Mall shopping centre in Walthamstow, east London.
Party members have until Monday evening to return their ballots for Jeremy Hunt or Boris Johnson.
The prime minister is expected to receive updates and discuss the security of shipping in the area.
The third in line to the throne is seen smiling in an England football shirt in images taken by his mother.
Jo Swinson or Sir Ed Davey will be named as Sir Vince Cable's successor on Monday afternoon.