Whether her name is on the deeds (Registered Title) or not she could still be entitled to a share of the equity if you split up years from now.
Her name cannot be on the Registered Title if you get a mortgage solely in your name.
If her name is on the Registered Title she will be a legal owner the same as you and be entitled to a share of the equity, not necessarily 50%, depending on how you own the property - as joint tenants or tenants in common.
http://www.lawontheweb.co.uk/basics/cohabit.ht m
You could make a cohabitation agreement but it must be done properly.
http://www.oneplusone.org.uk/marriedornot/Cont ractsCohabitation.htm
It would be a hard person who throws a partner out with nothing after living together as man and wife for years. Bear in mind that if she believes this is to be her proper and permanent home, and treats it as such, she may have a right to a claim.