16:22 is easily doable if your flight arrives on time, immigration queues are reasonable, etc.
17:31 would be safer, of course, if you want more contingency for delayed flights and such. St Pancras isn't the worst place to have some time to kill (at least, the pre-security part). Some decent shopping and eating options.
Depending on your destination in Paris, the most hassle-free option might be just to book a connecting BA LHR-CDG flight. It shouldn't cost much more than the Eurostar by the time you factor in the HEX ticket.
Last edited by Reason077; Nov 8, 2011 at 2:52 am