I want to cancel my iMac order

Yes it does, assuming it was purchased directly from Apple. If you bought it from a reseller then you must deal with that reseller. You can take it back to the Apple Store or call AppleCare and tell them you want to return it and they will help start the return process. Remember if you bought it from a reseller then you cannot return it from Apple as you did not buy it directly from Apple!!!!!

This is a user group, we are not Apple.

Login to the Apple Store and see if you can cancel from there otherwise contact Apple Support. Do it quickly.

You need to contact who you ordered it from.

We can't help with phone lines either. Be advised, delays of any kind may make the cancellation more difficult so be persistent and don't delay.

IF you bought it directly from Apple, if you cannot cancel the order you have 14 days to return it for ANY reason. Apple will pay the shipping both ways.

Does this 14 day return include an iMac that i have opened and used (within 14 days) and can i initiate a return online if i bought from an actual store (UK)?

Yes