Been on the phone to Tesla sales, and they said all cars entering the UK have been fitted with the noise maker, and that at the moment, the sound is disabled but will be enabled via a software update later according to law.
So even if you think you don't have the speaker, you probably have.
So far at least, the preparation for the speaker has only been seen on the M3, and no speaker actually fitted.
At some point soon it is likely to appear though simply because the USA has its own, similar, regulations and they take full effect earlier than the EU regs.