From what I've read, UK couples seem to have a lot easier time with communicating their wedding-related requests when they book directly through Disney rather than using a tour company like First Choice as a middleman, but I know sometimes they offer good deals if you bundle it with the holiday package.
Hopefully some of our many UK brides and grooms will come along and offer their insight...
I personally wouldn't go through an agent, as I want to be a lot more in control of my wedding and get to make the important choices etc.
I had a brief look at the Virgin packages, and lots of things jsut seem to get decided for you and are set in stone, whereas through DFTW pretty much everything is customisable.
I looked at booking a package through a tour operator before I decided on a wishes. The people I spoke to on the phone were useless, they didn't know ANYTHING! They told me I could book the WP at 12 months before - WRONG! They told me that I could book everywhere else whenever I wanted -WRONG! and they told me that I couldn't customise anything, even the bouquet colours! This was Virgin though, not First Choice. My advice would be to go through Disney though!