Arnaud, if you're getting the train from Portsmouth I believe you'll be coming into Waterloo. If that's the case, you could get the Northern line to Leicester Square, and then the Piccadilly line to Russell Square. If you're meeting people at St Pancras then you only need to stay on the Piccadilly for one more stop to Kings Cross

Has it really been five years!? Where's the bloody time gone?

BTW everyone, if you're planning to make extensive use of the Underground while you're here, I would strongly advise you to print off or grab one of these if you're unfamiliar with it - http://www.tfl.gov.uk/assets/downloa...d-tube-map.pdf