Britain urged its nationals on Saturday to consider leaving Russia after Moscow's decision to invade Ukraine.
"If your presence in Russia is not essential, we strongly advise that you consider leaving by remaining commercial routes," the British government said in a statement.
On Monday, Britain advised its citizens against all travel to Russia due to a lack of available flight options and increased economic volatility.
Read more: Russia declares partial ceasefire to allow humanitarian corridors in Ukraine
When asked by foreign correspondents in London why Roman Abramovich, the highest-profile Russian businessman in Britain due to his ownership of Chelsea Football Club, had not been sanctioned, Johnson said the government had to move carefully.
"None of us want to live in a country where the state can take your house off you without a very high burden of proof and due process," he was quoted as saying in the interview conducted on Friday by Italy's La Repubblica and other newspapers.
"There's no point saying, yeah, we're going to go after him, and then you come up against the brick wall of lawyers. So we have to get it right. We're also trying not to just make this about one individual," Johnson added.
Britain has been criticised for not moving as quickly with sanctions against individuals as the European Union, the United States and Canada.
Read: Russian forces seize Europe's largest nuclear power plant in Ukraine
Johnson said amendments to Britain's economic crimes legislation due to be presented to parliament on Monday would help the government act with more speed.
"Top line of what our package on Monday will do is that the measures that you have against individual oligarchs in Europe will essentially allow us to catch them too," he said.
Britain on Thursday sanctioned two more Russians – industrialist Alisher Usmanov and former deputy prime minister Igor Shuvalov.
Abramovich said on Wednesday he would sell Chelsea Football Club and promised to donate money from the sale to help victims of the war in Ukraine.