The BA A318 do NOT have ACTs.
The main fuel tanks are the same size as any small Airbus, about 19100kg at 0.800.
The ACT as fitted to A319 320 321 does not fit in an A318 as the cargo doors are smaller. Airbus has developed a new one that does, but it was not available in the timescale that BA wanted, so I assume that there are no A318 with ACTs yet.
There is a description of the A319 ACT on the Airbus site.