"I would have thought that Sentinal was a more appropriate platform, given that the immediate issue in and around the Crimea is the disposition of Russian ground forces. Or is this part of a wider effort to assure the Baltic States that their lack of organic air defence is ok with us and we will monitor any perceived Russian air threat?"
To reiterate... The Sentry is a NATO asset! When NATO says go then the Sentry goes. As the UK Component of the AEW&C Force, the Sentry will no doubt join the E-3As from Geilenkirchen in providing surveillance from NATO airspace as it always has done! This is not the first time that E-3s have deployed to Poland. (And yes, the MOD has to apply to NATO for a Sentry for National tasking - something that has upset many VSOs over the years when they have been told that they cannot have a short notice Sentry for their exercise).