These are three of London's eighteen railway stations: Euston, in yellow; St. Pancras International, in green; and King's Cross, in red. In summary:
>Euston - the country's busiest passenger terminal, and mainly handles traffic to the north-west.
>St. Pancras - Seventh busiest station in London, and its international train terminus; also handling domestic passengers from the Midlands and commuter traffic from the south-east.
>King's Cross - Eighth busiest station in London, and handles traffic from the north (Yorkshire, Tyneside, Scotland).
>EUS has its own London Underground connection; STP and KGX also interchange with the London Underground via a shared megacomplex.

See, the problem is - Euston will be London's terminus for the domestic High Speed 2 railway. However, because of costs (of course), and the borough of Camden complaining about construction works, High Speed 2 will not directly connect with the existing High Speed 1 infrastructure that connects to St. Pancras (i.e. the Channel Tunnel Rail Link). So - what do you think is the best solution for passenger interchange between HS1 and HS2?