The media has been putting forward some question as to who becomes the Official Opposition should Labour split.
Lets take an extreme example. Lets say, first off, that a Labour MP resigns. That makes the math a bit easier.
Next, lets say the party split. Not in half, but three ways. One faction has 77 members, one has 76, and one has 75.
Who is the official opposition?
The one with 77 seats.
The UK uses the same westminster system we do. The largest party that is not the government is the official opposition. If you have one more seat than the official opposition, and your party is not the government; congratulations, you are the new official opposition.
It does not matter if you contested the last election or how many people voted for you. All that matters is how many seats you currently have.
It really is just that simple.