pee, thank you. Kuopio is as far from Spb, as Tampere and it even takes less time to reach Tampere because of the 100-120 kmh highways almost all the way.
Don't you know if there are any plans to convert Kouvola-Uitti to civil or military/civil airport? Savonlinna is great place to come, there are thousands of rent cottages out there, and it is only 300 km. from Spb. The only real option, imho.
EISNN, 400 km by bus is not huge problem but border passport and traffic control longs 3-7 hours sometimes. Why are you talking about HEL?
