I checked with the Electoral Commission and they confirmed you don't have to mark all parts of the ballot.
If you just want to vote for a party and don't see any point in wasting your time on the local vote because the same old party hack always wins it....then you can do that. Just leave the local part of the ballot blank. That vote will be counted as informal. But your party vote will count toward seats for the party you support.
I was motivated to ask because where I live the same party almost always wins and it doesn't matter who I vote for locally.
It's the party vote that really counts. All the local votes do is decide whether a party has a local MP or a list MP.
Some parties choose their list MPs wisely and I tend to vote for them. National is particularly undemocratic about selecting list MPs....which seems odd as this is one their main criticisms of MMP. That seems perverse.....doing the very thing they claim they don't like. Anyway....I won't be casting another local vote.
We don't have to.