I'm not up to date with all the legalities, but there's quite a lot of legal things that need sorting out if you don't. It matters even more if you go abroad.
Things like next of kin, inheritance, tax, rights & finances if you split up etc etc
Even having a will doesn't cover all of that. It probably is cheaper to just go to a registry office than to pay a solicitor. You can always just nip in one lunch time, no need to have a big wedding.
We moved to the US 3 years ago & have had to make sure we have wills (even though married) & other paperwork to protect ourselves/each other.
Currently, if your dp died, ANY member of his family could claim all his property, which could then mean a lengthy legal battle, followed by paying 40% inheritance tax on anything that you did get. If you were married you would automatically get his 'estate' and not have to pay tax. That's quite a significant difference, at a time when you'd be dealing with grief & being a single parent. Do you want to be in that situation, or put him in that situation?
If one of you has far more assets than the other, then you can draw up a will which indicates how things should be split upon death. Pre-nups aren't legally binding, but are often used as a 'guideline' should the couple split.