This might be much more than you need, but Webasto is a renown brand for preheating engines (and interior).
http://www.webasto.co.uk/am/en/am_automobile_3177.html
If they have something for the Mini it won't be cheap, but idling an engine until it's warm on a daily basis is a very bad idea as can be read in the referenced thread.
http://www.webasto.co.uk/am/en/am_automobile_3177.html
If they have something for the Mini it won't be cheap, but idling an engine until it's warm on a daily basis is a very bad idea as can be read in the referenced thread.