Some of those units will be moved to the east to fight in the Donbas, but many of the forces will need "significant replenishment" before being deployed, the MoD has said.
A briefing note added: "Any mass redeployment from the north [is] likely to take at least a week minimum."
Ukrainian forces have been retaking areas in the north previously occupied by Russian troops.
They say they are discovering more and more atrocities.
Bucha, in the outskirts of Kyiv, saw bodies strewn in the streets.
Ukraine has been devastated by the ongoing war
ALEXANDER ERMOCHENKO
Ukraine's Zelenskyy his called on the West to do more in the fight against Putin
UKRAINIAN PRESIDENTIAL PRESS SER
It prompted global outcry and accusations of genocide.
Overnight, Ukraine President Volodymyr Zelenskyy said that in the town of Borodyanka, the situation was "significantly more dreadful" than in Bucha.
It comes as Prime Minister Boris Johnson is set to meet German Chancellor Olaf Scholz as they look to discuss how to help European countries wean themselves off Russian gas following the attack on Ukraine.
The PM will host Mr Scholz at Downing Street, with a press conference planned for the afternoon.
Mr Johnson is expected to offer assistance to Berlin, which is still heavily reliant on Russian gas, to reduce its dependence on Moscow’s energy exports in a bid to starve Vladimir Putin’s war machine of funds.
In a tweet, Mr Johnson said: “Looking forward to welcoming
@BundesKanzler Scholz to the UK today.
“I welcome his principled determination to end dependence on Russian energy. How we respond to Russia’s invasion will define the international order for years to come. We cannot let Putin’s crimes go unpunished.”