www.1837online.com
www.ancestry.co.uk
These aren't free, but you can buy credits so you can see how much you are spending. Some of the census' are free, so you won't have to pay for those at all.
www.genesreunited.co.uk
I have recently found three distant cousins using this site, one of them has traced one branch of the family back to 1692!
Remember to start by asking your relatives all they know about their family. They may well have photos, birth, marriage and death certificates or even grave papers.