See this: https://www.londonelects.org.uk/im-voter/how-vote
And note that "You have 2 choices for Mayor" so you don't have to confine your vote to the usual two big parties for tactical reasons. If you vote for a "small" party with little chance it's not wasted, you have a second choice as backup.
Preferential voting: It's a good thing!
> If a candidate receives more than half of all the first choice votes they are elected immediately
There is far more choice on the centre and left than there is on the right. The two leaders in the polls are neck and neck.
If you are centre-left, you may wish to be defensive and put your vote for the most likely to win, and your second on the one you believe in.
It still sucks.
What makes you think that one of the candidates will get 50% of the vote? And what makes you think that allocating the other voters differently will change that?