Yeah a first in all three years.
They can't judge how much effort you put in in first year, they can only go by what the results on the page tell them. In addition they probably want people who always put in effort. Even if you could demonstrate that you didn't put in much effort, not putting in maximum effort looks lazy.
If you have a good excuse or you've shown a consistent improvement, that may help your case but obv, a good result in first year is desirable. Especially if you're in final year and they're working off your predicted grades.
Extra curriculars don't matter at all. I'm not sure about work experience, I suppose it helps although the MPhil Finance is for people with little or no work experience.
But obv, this is all based on my opinion/experience, I don't know what the exact mindset of an adcom is.