I did French, German and Italian at A Level, going on to do French, German and Japanese as my degree (with a couple of optional modules in Dutch).
I would definitely advise against Japanese single honours when you've got 2 A Level languages. After the effort you've put in already it would be silly to drop them and, as previous posters have said, French German and Spanish are still the top 3 in demand languages by businesses in the UK.
That said, I can't remember where Portuguese fell, Mandarin was around 5/6, Japanese was 10th.
I would still say that Mandarin is significantly more useful than Japanese. Although that's not as important when you've combined. I would probably say Mandarin seems harder, but I guess that depends where your talents lie!
Regarding fluency - I would agree that I am not fluent in Japanese after graduation. However, after my year abroad I was 4 marks off passing JLPT 2, so I imagine its fair to say that I am now at JLPT 2 standard, which is the standard most employers ask for. So in terms of job prospects, my not feeling fluent isn't really relevant. Also, I graduated Newcastle with Level C Japanese (normal for someone who came in as a beginner) - 2 of my year group skipped ahead to Level D Japanese which aimed to get you to JLPT 1 standard by graduation. So those two students passed JLPT 2 while in Japan. Trying to skip to Level D was the reason I took the JLPT but yeh... 4 damn marks!!!
I also know that at least 2 students from Chinese skipped into the Level D class.
So I wouldn't say it's impossible to achieve fluency.
I don't know about the people you've spoken to who did single honours Japanese, but the majority of my classmates were studying Japanese as their only language and didn't have much experience of studying languages, so they did seem to struggle and not reach as high a level as I did. Japanese, more than any other language, does seem to attract more "non-language" students - by which I mean those who haven't done A Level languages and don't necessarily have the aptitude/natural skill for learning them.
Obviously you're in a good position having done 2 A Levels in languages so you'd probably not find it as challenging as some
Also, why not consider the 3 languages like I did? French, Spanish and then either Chinese/Japanese/Portuguese?
You still get the same amount of language classes as single honours students