Kevin,
Our family went and rode last year and had a blast. We flew into Manchester Airport, rented a car and cruised around for 3 weeks.
I wouldn't be put off by considering doing the WHR on your own and not w/ a package tour. You can step on British rail service right at the Manchester airport and shoot over to say Bangor, then rent a vehicle. If I was to do it again, that is what I would do. Your own wheels give lots more flexiblity to visit other lines, the many castles she wants to see, small town shopping, etc... And driving on the "other" side of the road keeps you on your toes so you don't get bored. The whole area is very visitor friendly.
I wouldn't have missed riding the Ffestiniog, the Vale of Rheidol, and others..all an easy drive from the Portmadoc area. The good folks at Boston Lodge were kind enough to put me to work on engines for several days. (I guess you have to be nuts to cross the ocean to do a boiler wash.) Anyhow, it was loads of fun. I wouldn't be intimidated by considering your trip w/o a cruise and yes my wife had a great time too. She loves to plan trips and she did a super job with that one.
Be glad to help more if you are interested.