There are lots of things about tax and inheritance which happen automatically when you marry in the UK. Most of them (not all) can be done separately if you're in a relationship but not married/civilly partnered.
I mostly don't think the law needs to change for that, but I do think that inheritance laws should be extended include close relationships like siblings who live together, and I also think that some people who live together wrongly think that gives them rights over property or children which they might not have - I think it's fine not to marry if you know what that means legally when you buy property or have children - but many people don't. I'm not sure how you force people to have knowledge, though.
I have a German colleague who said the main reason he and his spouse married was because of the tax benefits.
I'm not sure I should get any benefits because I'm single (although I quite like my 25% council tax discount.) I do think I shouldn't have to be charged extra for being single when I go on holiday and so on. There are some benefits to being single in that I mostly don't have to do things I don't want socially, but there are also a lot of things which are crap about being single, and paying single supplements are one of them.