As has been mentioned, there is little consistency between airlines when it comes to the concept of "premium economy".
Hopefully, you'll be able to report back so that you can get some meaningful advice and information.
I did find this photo of a "Thomas Cook premium economy cabin", perhaps this will help:
http://www.thedibb.co.uk/forums/show...3&postcount=21
(not great, but even so, considerably wider* than the seats in the economy section behind - and the seat arms do appear to be 'hinged', a big plus)
*Edit to add - now I've looked at the photo again, I don't believe they are 'considerably wider' after all...
Alas, it seems extremely unlikely that you will be flying bmi. But I would like to point out this airline to you as a consideration for future trips as it offers what I consider to be one of the best premium economy seats of any airline.
The seats used for their long haul flights used to be those used in their business cabin - I flew from Manchester to Las Vegas and back when they were up at the front of the aircraft and found them to be immensely comfortable.
Now they have been moved further back to the premium economy section (replaced with more 'advanced' seats in the Business class section). However, the seats are essentially the same, they just have less legroom than before.
Here's a photo:
http://www.flightglobal.com/assets/g...x?ItemID=19858
The seats are 21inches wide (I think this is similar to the seat width offered by Virgin)