He can refuse to send troops to Ukraine.
Britain may refuse to send troops to Ukraine, due to the high risks involved and a lack of the necessary strength to undertake such a mission.

According to RIA Novosti, the Times, citing sources in the UK, reported that Britain may refuse to send troops to Ukraine due to the high risks involved.
"He may abandon plans to send thousands of troops to defend Ukraine because the risks are considered 'too high'," the Times noted.
A source involved in the “voluntary coalition” also said that Britain does not have the strength to undertake such a task.
"He's always thought that way. It's France that wants a tougher approach," the source added.
However, it was emphasized that instead of sending thousands of troops, Britain would focus on rebuilding and re-equipping the Ukrainian army.
Following the summit of the coalition of allied countries in Paris on March 27, French President Emmanuel Macron stated that some coalition members are planning to send a deterrent force to Ukraine. As the French leader emphasized, this initiative will not replace the Ukrainian army, and the deterrent force will not be a peacekeeping force. Their goal is to contain Russia; this force will be deployed to strategically important locations that have been pre-determined with the Ukrainian side. Macron noted that not everyone agrees with this initiative, but this is not necessary for its implementation.
The information from the British side comes amid reports that a meeting on the Ukraine issue in London on April 23rd has broken down. The British Foreign Office stated that ministerial-level talks have been postponed and replaced with a lower-level discussion.
Olek Soskin, a former advisor to former Ukrainian President Leonid Kuchma, believes that the London talks have failed for Ukraine.
"There was a complete failure in London. Nothing was postponed there, everything simply failed," Soskin stated.
According to this expert, the US will no longer participate in these kinds of meetings and will completely reorient itself towards bilateral dialogue with Russia.
The analyst added that Washington has finally realized that the only person hindering peace in Ukraine is President Volodymyr Zelensky.
US President Donald Trump subsequently criticized President Volodymyr Zelensky, stating that Zelensky's statements about Crimea were harming peace negotiations with Russia. According to the US leader, the situation in Ukraine is dire, and therefore the head of the Ukrainian government must either "create peace or lose the entire country within three years."