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You can get the adapter after the fact. It's no more expensive than having ordered it that way in the first place. And if you have an Apple Store close by (I don't know if they exist in the UK) they'll install it, too.

There are three Apple stores in the UK now. Birmingham, London and Bluewater so far.

An external USB WiFi dongle would be cheaper than getting Apple to install the part. There is no point in trying to install it yourself. Whilst the procedure isn't too complicated (paint scrapers excluded) it is difficult to get the parts and if you've got a mini without Bluetooth and WiFi then you're missing the mezzanine board that they plug into.