As a CO Gold you'll be Star Alliance Gold. That gets you free access to any *A Gold lounge when departing on any *A carrier. You can use the AC/SK "London Lounge" if flying CO out of T3. If SQ is still in T3 (don't recall) you can also use their smaller but very nice lounge as well. If CO moves to T1, which I'm sure will happen sooner or later, then the shared *A Lounge and the separate BD lounge are available.

The only time you can't use a *A lounge based on your CO G *G status is if you're using a USA-based UA, US, or I expect also CO lounge on a purely domestic itinerary. Since UA, US, and CO sell lounge memberships, and have some many North American-based elites, their lounges don't admit on pure-domestic itineraries. US Clubs define Canada and Mexico as still "domestic" for purposes of denying access. But UA lounges do admit USA-based-program *G members on Canadian and Mexican itins as "international". Also as a *G, if you physically have airside access to where there's a SQ, AC, LH, or other non-UA/US/CO lounge, you can access on any *A itinerary, not just an international itin. For example, the new LH lounge in IAD Terminal B - any UA 1P/1K, any US GE/PE/CP, and as of 10/27, any CO Gold/Plat, will have access when flying any *A itin out of IAD. If you have your card - LH Lounge Dragons are merciless about having a valid current-year card with a Star Gold logo on it.