Normally you would get a company who will do the complete job from design, through any planning needed, to the final installation.
Like with any other building job it would be best if you could find someone local to you who has already had it done, so you can ask who they used, what they were like, and possibly see the finished job.
Here's one in Coleraine that does the complete job.
http://www.gilmoreplumbing.co.uk/bathrooms.htm
And a couple further away in Belfast.
http://www.rjspatterson.com/services/#tab-1-2
http://www.jrgroves.co.uk/