Please explain next example:
My neighbour is a photographer; let's ask him for (!) advice about colour films.
We had (!) fish and (!) chips for (!) lunch. That doesn't sound a very interesting lunch.
We'd better go by (!) taxi—if we can get (a) taxi at such (an) hour as 2 a.m.
Why do we not use articles where I put an "(!)"? What rules apply in these cases?