Margaret of Anjou should have had it all. Her husband, King Henry VI, was in her possession. She had a massive army and the road was now clear to London. But what she hadn't reckoned upon was the Londoners.
From their point of view, the hard won Act of Accord had ended the brutal Wars of the Roses. Then their foreign queen had broken the peace. Moreover, she'd done so with a massive army of Scots! (England and Scotland had spent several centuries fighting each other by this point.)
Said Scottish army had been looting and pillaging their way south of the River Trent (as per the agreement hashed out between Margaret of Anjou and Mary of Guelders). And now the queen wanted to lead her Scottish army into London.
She found the gates barred against her. King in tow or not, she was NOT bringing a huge Scottish army into England's capital city.
Margaret didn't have the supplies for this. She could not camp outside London's walls for weeks on end. As she began the withdrawal north, towards the city of York, much of her army realized that the game was up. They broke away and marched back over the border, taking their plunder with them.
Meanwhile, the two bodies of the Yorkist army, headed by Edward and Warwick respectively, joined forces. When they appeared at London's gates, they found them opened wide.
The Earl of Warwick strode into Westminster and proclaimed his teenage companion to be their rightful monarch. Though it was only five months since Parliament had refused to support the same declaration from Richard, Duke of York, the nobles were more amenable to the idea this time.
Which gives a measure of just how upset they were over Margaret's actions.
Hurriedly crowned Edward IV, the eighteen year old Plantagenet wasted no time in summoning the full strength of southern England to fight for him. Fully aware that this was going on, Queen Margaret was doing the same in the north, on behalf of Henry VI.
King Edward led his forces north in terrible weather. He'd managed to attract around 30,000 men. Already in Yorkshire, the House of Lancaster got to choose the site of the battle. Approximately 35,000 men turned out for Henry. They took the high ground near the village of Towton and waited.