Aha. Turns out that Airbus did a bit more than one Tough-and-Go.
From
LCY "Runway News"
Following the successful evaluation flights in May
2006, the Airbus A318 returned to London City Airport
in October to perform a series of compatibility tests.
Approval for London City Airport will make the Airbus
A318 the latest generation airline in the 100-seat class
to be able to operate out of the Airport. With the widest
and most comfortable cabin, the A318 will bring higher
comfort to business routes operating out of the Airport.
During this visit the A318 showed that it could meet the
steep approach and low noise level requirements at
London City Airport, confirming itself as a good neighbour
at inner city airports.
So the 318 did two sets of tests. That the 319 would only do a very limited set of tests is therefore not surprising.