Force of habit, probably. TC is probably so used to getting most London TMA departures straight 'off the deck' that they probably forget that some of them have already worked 'TinPot Radar'. Remember, Thames Radar was not always in the same Ops Room so perhaps it was a case of 'out-of-sight, out-of mind'. Hopefully the 'disowned orphan' has finally been permanently adopted and embraced within the fold...