One of the benefits of conformance at LHR T5 is that they know who’s in the building and therefore who not to chase. Otherwise the act of checking in is in effect telling them you want and intend to fly, so they are expecting you, so to speak, and will wait until standard close time if you’re not there. If there’s a chance you won’t be flying, it’s best not to check in for this reason. If plans change once checked in, as CWS mentioned, a quick call to get yourself offloaded is both polite and recommended.