The difference between Present Subjunctive Conditional and Past Subjunctive Conditional?

I am trying to understand what is the lexical difference between next sentences:

If I went to London, I would see the Thames

and

If I had gone to London, I would have seen the Thames

Answer

If I went to London, I would see the Thames.

can mean two things:

1 it describes a repeated past activity.

2 it describes a non-past irrealis. The speaker is not actually going to London.

and

If I had gone to London, I would have seen the Thames.

This is a past irrealis. The person did not, in the past, actually go to London.