Here are links to the railways I have visited. They are all a fantastic day out and every one offers something different!
Keighley & Worth Valley Railway - http://www.kwvr.co.uk/
Nene Valley Railway - http://www.nvr.org.uk/
North Yorkshire Moors Railway - http://www.nymr.co.uk/
East Lancashire Railway - http://www.eastlancsrailway.org.uk/
Great Central Railway - http://www.gcrailway.co.uk/
Didcot Railway Centre - http://www.didcotrailwaycentre.org.uk/
West Somerset Railway - http://www.west-somerset-railway.co.uk/
Llangollen Railway - http://www.llangollen-railway.co.uk
Online forums are a great source of information and banter. Here are the forums to which I am a member.
National Preservation - http://railways.national-preservation.com/forum.php
Rail UK - http://www.railforums.co.uk/
WNXX Interactive - http://www.wnxxforum.com/interactive/index.php
The Works - http://intheworks.co.uk/forum/index.php
NYMR Unofficial Forum - http://nymrunofficialforum.createforumhosting.com/