St Pancras International is a railway station in London, not an airport, so an airport-style lounge search here needs a quick reframe before it goes anywhere useful. QQS is the code attached to this Eurostar terminus, and the honest starting point is that whatever lounge exists belongs to the trains, not to any airline.
That said, St Pancras is not lounge-free the way a small regional airport might be. Eurostar operates lounges for premium ticket holders at its terminals, a benefit tied to Business Premier fares rather than a card network or airport-style membership, and arrangements like this can shift, so check your ticket class and Eurostar's current terms before you travel rather than assuming a standard lounge model applies. This is a genuinely different access system from anything you would find at an airport, built around train fare classes rather than airline status, elite tiers, or a Priority Pass style membership card.
For an actual airport lounge experience in London, you need to leave St Pancras and head to one of the city's real airports. Heathrow is the obvious choice, with an extensive lounge landscape including British Airways Galleries lounges and a wide range of airline and card-network options across its terminals. Gatwick also carries its own lounge lineup for departing passengers, smaller than Heathrow's but genuinely airport-based, with Priority Pass and airline options in its terminals. Both operate on the familiar airport model, security, airside lounges tied to fare, status, or membership, that a rail terminus like St Pancras does not share.
If you are travelling by Eurostar, look at your fare class for St Pancras lounge access; if you are flying, plan your lounge expectations around Heathrow or Gatwick instead, and confirm current terms with whichever operator, rail or airline, actually applies to your journey before you travel.