I would go as far as to say that it is IMPOSSIBLE to do it for free. Census info, certificates, subscriptions to websites etc all cost money.
You could probably get someone to do the census lookups for you but as spudqueen says, at some point you will probably have to buy certificates and they cost �7 each.
You could try www.rootschat.com which is not a site where you can look for info yourself but you can post requests (for free) and others will do look ups for you.
www.genesreunited.co.uk is another good site with lots of helpful people willing to do searches, but this will cost you �9.95 to join.
If you get back to the mid 1800's and earlier, you can use www.familysearch.org they have the 1881 census and the IGI (both free). The IGI (International Genealogical Index) is an index of baptisms, marriages and deaths (less deaths), some taken from actual parish records and some submitted by members (less reliable). The coverage isn't complete but you may strike lucky.