"Not being super clued up with the way my American cousins implement their particular flavour of democracy, I wanted to float an interpretation to see if I "get it"..."
The U.K. and U.S. do things very differently, so it's hard to make a parallel. Yours doesn't really apply.
"Is this essentially the same to us Brits voting some nutter into power (some party that is close to the far left / far right), and then when the nutter Government tries to pass the budget which has us spending £20bn on a wall the MP's don't let the budget pass? However spunking £20bn on a wall was in the party manifesto in which they campaigned on."
Not now. That happens too, but at this point, it's two different groups disagreeing. The most analogous thing in British politics is when two parties that are in coalition disagree or one party has a major split. However, even this is less antagonistic because the coalition implies that the parties did agree at some point on their policy, but have separated. In this case in the U.S., one side campaigned on the policy while the other side campaigned on not allowing the policy to proceed, both receiving enough votes from somewhere to get them a position of power. In this case, this happened across multiple elections because terms overlap, but it would also be possible under the American system to have something like this happen in one election. Each side feels it has a mandate to the voters that voted for them to provide for or block the policy. The people who are in charge can't pass the policies that they want because they lack the votes. However, in the U.K. this would usually lead to a vote of no confidence and another election. The U.S. does not allow that. The executive and legislature are independent, and neither can remove the other. So they continue to have a disagreement until someone changes their mind or they just ignore the topic and do other things. Or in this case, they choose not to do anything at all.