Essentially there's two variants:
Audax; you have a distance and some directions on paper/gps, with checkpoints and fastest/slowest speeds. Otherwise you're on your own. The cost is usually minimal Have a look at Audax UK:
http://www.aukweb.net/ Vernon on here does Audaxes on a hybrid. Peak Audax is the closest organiser to you I think;
http://www.highergrangefarm.fsnet.co.uk/PeakAudax/calendar.htm
Sportive; organised event with signage, mechanical support, feed stations, often with timing chips, etc. These cost more (£10-60) but have more support. It's not a race, although many treat it as such
It's personal preference. I've done a couple of Audaxes and about 6 sportives this year, plus one last year (Wakefield Bikeathon 26 miles). I've another 3 sportives planned for this year; Otley Sportive this Saturday, Etape Pennines (Oct) and Chemo Classic (Nov) plus the BHF charity Manchester-Blackpool overnight ride.
The Road Rides, Audax and Sportives section of the forum will give you a start:
http://www.cyclechat.net/forums/road-rides-audax-and-sportives.45/
Alternatively, there are a number of informal rides organised on here - again, check the
http://www.cyclechat.net/forums/cc-informal-rides-and-events.47/ sub-forum.
Pick an event and give it a try!