去吃午饭的英文表达有多种，以下是一些常见的表达方式：

I am going to have lunch.

I'm going to lunch.

I have to go for my lunch now.

Let’s go to have lunch.

I'm out to lunch.

It's time for lunch.

Let's eat lunch.

根据具体的语境和需要，可以选择合适的表达方式来描述去吃午饭的情况。例如，在正式场合或需要详细说明时，可以使用“I am going to have lunch.”；在较为随意的场合，可以使用“I'm going to lunch.”或“Let’s go to have lunch.”。