Tighty mentioned some concerns to me about possible problems purchasing & re-selling Estonian Kroons in the UK.
I can"t find anything specific on this but it is a "minor" currency so there may be some places that will have trouble dealing with your requirements.
Estonia is set to join the Euro come January 1st 2011 and the Kroon will go the way of the France Franc and the Deutschmark. This may add to perceived difficulties in re-selling.
My suggestion would be to exchange any surplus local currency at Tallinn airport before catching your flight back.
EDIT: Looks like you can buy and sell Kroons through the
Post Office.