This decision was made because in the United Kingdom there isn't sufficient demand for the two models to justify its sale, so say goodbye to them, at least for now.
A Subaru spokesman said the following, "The new Impreza has been under evaluation in the UK for 12 months, but the decision has been taken not to bring that car to the UK because of insufficient demand.”, adding, “Currently we have no plans to bring in further [WRX STI] stock after the current stock sells out – which should takes us to early summer depending on demand.”
So if you really want one, you have to go to a Subaru dealer and buy one they will soon be gone. But Subaru won't just leave the United Kingdom, the BRZ, the XV, the Forester and the Outback will still be available to order.
Source: AutoExpress UK