24-hour security and off street is definitely better than leaving your bike locked to some street railing, but you might like to consider the London Bridge cycle parking facility. This has much better security as you can lock your bike and it's within secure storage for which you have to provide a receipt to enter. You do have to pay a couple of quid but it's worth it I feel for a few days. I know it's not quite as convenient for St Pancras but it's only a short hop - depends how much you value your bike. Plus there's a bike shop adjacent which can be handy for repairs and stuff.
A nice touch with the St Pancras cycle parking is a rail to leave your heavy lock, see photo:
located at http://twitpic.com/5ca0lj
Link to London Bridge cycle parking facility: