Who you vote for vs who the guidelines say you should vote for are 2 different things. And are treated by such even by MPs themselves.
I quite understand I am meant to vote for my MP and that is the mechanism by which my vote is registered.
But what actually drives people to vote may differ wildlely and lots of people don't really know their MP so choose their vote based on what they know of the Party and of its leader. They do vote for Prime Ministers, using the loal MP mechanism to do so.
What's more, when expedient, the Parties themselves encourage this. In the 2019 election, my local MP encourage people to vote for him because "only Boris can get Brexit done". He was actively encouraging people to give him a vote based on their desire to see Boris as the PM. It's the reason certain MPs went public for Johnson claiming he was 'the ONLY one who had a mandate to lead.
There is a reason why the PM job gets so much news coverage: afterall, if it was simply a role of 1st among equals, as it is meant to be, it would not attract that level of attention. Ditto, there is a reason the PM candidate often spends so much time travelling the country to help encourage votes. If they didn't want voters to consider this when voting, they'd stay away.
Governments want voters to vote for a PM when they think it will help them. Now we get to people rejecting a PM, asking for a GE and they want to fall back to the letter of the law as a reason not to. Cake and eat it, springs to mind.