Not London - but why would a pelican crossing suddenly acquire a aerial
on
the top?

Was it crossing as part of its annual migration, and the aerial was
tracking it?

Or was it a pelicon?

The original acronym might have been pelicon but pelican is the accepted
spelling these days. As in "The Zebra, Pelican and Puffin Pedestrian
Crossings Regulations and General Directions 1997" for example.