Hello Jim. Welcome to the forum from another UK Byoac'r

3/4" (19.05mm) t-moulding is fine to use on 18mm MDF, you will only have ~0.5mm overhang either side which will barely be noticeable. If you really are concerned about the overhang you can trim the edge, take a look at [
this] thread (which is coincidentally right below this one at the moment in time.
Note you wont be able to trim the t-moulding if you are using chrome/gold as it is generally white t-moulding with a layer of metallic chrome/gold film.
I have used 3/4" chrome t-moulding with 18mm MDF successfully on a couple of projects so far.
I bought mine from
http://t-molding.com/store/home.php. They have a good selection and shipping from the US has always been fairly fast.
Edit: Must have posted at the same time Jimbo!