I think we need more information to answer this.

If for example you had an issue with being IDBed the issue is with the airline. I find it better to worry about that IF it happens as it's relatively unlikely.

I find it hard to think of any issue that could arise at LHR that wouldn't sit with the carrier. I mean sure you could be missing your eTA and that's not the airlines fault.. it's also not the CTAs fault.