As you've now opened it, you won't be able to swap it, so you might as well use it, which you can - an intermediate switch can be used as a 1-way (which is what you need), or a 2-way, which is how landing lights are usually wired, i.e. 1 light, 2 switches. The only time you actually need an intermediate switch is if you have more than 2 switches controlling 1 light.
Anyway - I'm going to assume that the dimmer you want to replace is in a 1-way setup, i.e. it's the only control for that light, as this is how dimmer switches are used nearly all of the time.
A 1 way switch has 2 terminals, and operating the switch connects or disconnects them.
To visualise how an intermediate switch with 4 teminals works, imagine the terminals as the 4 corners of a square, so you've got Top Left, Top Right, Bottom Left & Bottom Right.
With the rocker in one position the connections made are TL-BL and TR-BR, and with it in the other position the connections are TL-BR and TR-BL. If you actually draw this little sketch on paper you will, hopefully, see that if you use, say TL and BL, then in one switch position they will be connected, and in the other they won't, which is all that you need.
The only problem is that it's been so long since I wired up an intermediate switch, I can't remember which "corners" are L1 and which are L2, i.e. is it
[L1]==[L2]
[L1]==[L2]
or
[L1]==[L1]
[L2]==[L2]
so unless you have a multimeter which you can use to work out which terminals are connected when, or someone else here posts and says "it's always <this one>", then you'll have to do it by trial and error, which will mean to-ing and fro-ing to the CU to turn the power on and off. Please don't be tempted to connect or disconect any wires to the old or new switch with the power on.
Comforting note - there is no way to connect the switch up so that is dangerous - if you picked any two terminals at random, there are only 3 possible results.
1) It works fine
2) It works, but "upside down", i.e. lights on when the switch is up
3) It doesn't work at all
So turn off the power to the lighting circuit, and remove the dimmer switch. You should see two wires connected to it, 1 red, and 1 black, possibly with a bit of red tape or red sleeving on it.
Connect them to, say, the two L1 terminals in your switch, turn the power back on, and carefully holding the switch steady, work the rocker.
If you have Result #1 job done apart from turning the juice off again and screwing the switch in place.
If R. #2 you can either swap the wires over, or turn the switch upside down.
If #3, you'll need to move one of the wires to one of the L2 terminals and try again.
