Where can I play against specific openings? Is there a website where you can play against real people with an opening already installed by default? For example, I want to play against the English opening, but my opponents rarely play it, so I want to practice somewhere.
Lichess also has several thematic tournaments throughout the week.
As an example take a look at this thematic Italian game arena tournament:

When you open the tournament, the opening is mentioned in the upper left corner if there is one:

It is a link so you can click it to see the mentioned opening.
Alternatively you can create your own tournament with a opening of your choice. For that just go to the create tournament area and set the starting position:

In that box you have to place a FEN string. For the English opening considering only 1 C4, you can use the following fen:
rnbqkbnr/pppppppp/8/8/2P5/8/PP1PPPPP/RNBQKBNR b KQkq - 0 1
Regardless you can create any fen of your choice with the lichess analysis board tool or the opening explorer