I think this incident provides a perfect example of the dangers of volunteering that you are "visual" with another aircraft in an IFR environment.
By doing this you are risking exactly what may have happened here, a "clearance" - which is effectively "at your discretion" - to carry out a VFR procedure based on your assumption that your identification is correct. Using TCAS as an identification aid is even more hazardous imho than a controller's description (left 11 o'clock 3 miles) because he will - should - only give that if there is nothing else there that you might confuse it with while with TCAS you cannot be sure whether what you are looking at is the proximate traffic or one that TCAS is not showing. TCAS just isn't that accurate on bearings anyway, doing this is using it for a purpose it was never designed nor intended for, so DON'T!
I am never happy with confirming self-identified traffic to ATC - sure, look and see if you think you can see it but I never tell ATC I've identified it cos how can I be sure? That's his job. The word "controller" gives the clue. It is not our place to take responsibility for separation in an IFR environment, nor should they expect or encourage us to. Flying club procedures at an international airport are just going to end in tears.
imho it is a practice to be avoided completely.
And to do this at BCN of all places - with their shonky standards - madness.
Not, for once, blaming Spanish ATC entirely here, but their standards are generally pretty appalling and before too long are going to result in a heavy body-count. That much is a racing certainty.
Be careful out there!