Okcupid, times, guardian, telegraph, uniform dating, singleparents.com
The first is probably biggest out of those, works for some, not for others. The next three are lifestyle-based, and only for big cities.
Singleparents is horrible, from my brief look.
I've never looked at the uniform one
I'll see if I can dig out the post about lonelyseacaptain.com, animal costumes.com etc. but I'd avoid them