My little red book on the Integrale says that 9841 HF's were built between 1987 and 1989 so presumably they were on the market in Italy in 1987 if not in the UK.
According to Collins and Baker* the production of 5000 required for homologation was achieved by 1st of March 1988. They were in Italian showrooms by the end of October 1997 but it was March 1988 before UK journalists got hold of one.
* "Lancia integrale-----The Complete Story"