The win on Sunday earned them a play off game against the winners of London south west one for a place in NC two south west.
Valkyries had to wait until the stroke of half-time before taking the lead against tough opponents.
Then Jordan Wilshire scored after charging down a clearance kick.
Chloe Stopard added the conversion.
In the second half number eight Natasha Davison extended the home sides lead with a try from the back of a ruck.
Senior coach Mark Witcher praised his players for overcoming every challenge and for working hard to make the success happen.
He said: ‘The girls have stepped up and met every challenge put in front of them.
‘Winning the league and the chance of a second promotion in three years is a great achievement.
‘The girls deserve it because they have worked so hard.
‘I am backing them to go on even further.
‘The game in difficult conditions asked lots of questions of the side.
‘Reading were difficult opponents but the girls once again came out on top.’
At Hooks Lane Havant Ladies cruised to a 42-0 win against Witney Ladies.