Automatic border gates: compulsory?

Last time when I came back from a short trip in Europe, there were 2 options at Heathrow, the automatic border control gates and the traditional booths with Immigration Officers there.

I went to the booths and gave my passport to the Officer. He gave me a hard time because I went to him rather than to the automatic gates. I tried to make the point that these gates are optional but he kept insisting that they are now compulsory and when they exist, I must go through them and no alternative option is available to me.

He still let me pass eventually after carefully checking my passport but I still believe he wasn't right.

They are nowhere said to be compulsory. Gov.uk guidance everywhere says "you can use e-passport-Gates" to save time, avoid queues etc.