Their opponents, Canada, have exceeded expectations throughout the tournament but arrive at Twickenham as underdogs.
The team, which is semi-professional, was forced to crowdfund nearly a third of its World Cup campaign budget.
The King also sent his best wishes to both sides, writing: “Whatever the result, you have already done your countries proud.”
Saturday’s final has sold out, with the 82,000 strong crowd set to make history as the biggest ever for women’s rugby.
Earlier this week, England booked their place in a seventh successive Women’s Rugby World Cup final with a gutsy 35-17 victory over a stubborn French side.
England have played in eight World Cup finals, winning in 1994 and again in 2014 against their upcoming rivals Canada.