I've just discovered that I did the right thing in keeping on being taxed separately. I was going to be asked to be taxed as part of a married couple when I got back from France, but didn't get round to it. I'm very glad I didn't, because once your joint income goes over about £15K you start losing money and paying more tax. Under £15K you save on tax as a married couple. Between us we are getting on for £20K and would have lost £400 p.a.!!! So at least be taxed separately. Don't know about benefits, sorry.
If it is impractical to be legally married, but you are at all religious and it is important to your relationship, then you could ask for a church blessing - would that help? :)