Hi and welcome to MINI2. Changing the stereo is easier than it sounds, but maybe a tad more involved than you suggest. Really wouldn't let Halfords anywhere need it. See instructions in the link.
NewMINIStuff.com DIY Guides
Now do you have a Wave or a Boost stereo. Link below should help.
NewMINIStuff.com FAQ
Then with knowing this you will probably need the relevant one of these
NewMINIStuff.com
Mikey at newministuff is the daddy when it comes to this stuff
Good luck and enjoy the MCS.