Can a British citizen have dual nationality?

Russian citizenship requires similar qualifiers as most other nations; a right to live as a Russian begins with legal residency, then further qualifiers are added to validate the right to participate or enjoy all the rights and obligations of a Russian Citizen. A few of those items might be the right to vote, the requirement to pay taxes, retirement, the right to work within the country, etc.

Investment in Russia, in the form of private business, may be one of the paths to consider. Marriage to a Russian citizen may be another.

It should be noted that some countries expressly prohibit Dual Citizenship. This means in addition to a sworn alliance as a citizen of the country, the oath may include the rejection of any other nation and your rights to be part of that nation. Additionally, some countries mandate as a part of citizenship, the requirement for military service. This means that should you now be a citizen of Another Country, despite having a passport from a previous country, you can be conscripted (immediately) into military service, especially in times of a national emergency.

Your local Russian embassy can provide written details as to what current requirements are to apply for citizenship.