There are several reasons why Mr Cameron should not attend Russia's 70th anniversary of Victory Day in Moscow on May 9th, the foremost being that he may lose the election or be engaged in the opening days of coalition discussions. Likewise the Sovereign must remain in London to invite one of the party leaders to form a government.
However, Cameron ruled himself out some time ago as part of a US-orchestrated hissy-fit at Putin over Ukraine. America seems to forget too easily that Russia had 12,000,000 military dead compared to 400,000 US military dead, and it was Russia that made the British and Canadian -led invasion of Europe on D-Day possible. So far only the Czech president has told the US to get stuffed, banning the US ambassador from Prague Castle. Even long-lived Russian veterans of the war can't go on for ever, and this year is the last milestone anniversary for VE day, as 2014 was the last Normandy commemoration. The prolonged US sulk at having been wrong-footed over Ukraine should not eclipse Russia's true place as our vital ally against Nazi Germany.
We should, even at this stage, agree to send Prince Charles to Moscow, It is, after all, what he's for.