the bbc news website described the difference thus in 2005:-
"A partnership is formed when the second of the two parties signs the partnership papers. This is not necessarily a public ceremony or even an event that happens at the same time as the first signature. This flexibility means that couples can essentially become partnered in private, if they wish.
In contrast, a marriage happens when the partners exchange spoken words and also sign the register.
Another important distinction is that marriage as a word has religious connotations, even if a ceremony is only civil. Marriages can be conducted by Church of England clergy without any civil preliminaries being required. Civil partnerships are only conducted by registrars."