Circuit breaker says 100A, so I guess this is good. Is it fine to use an extension cable in a socket if I wish to run more?
Starting here. You do not want extension cables, just another point of failure and a fire hazard. I see below you are considering a PDU; good idea if you plan to expand further.
Your voltage at your outlet should be 240 ( I believe this is standard in the UK) so the amperage per S9 for you is anywhere from 5.9 - 6.1 Amps. ( I use 220 as my voltage for calculations and the Bitmain numbers from 1300-1340 W)
I see others have given 7 amps, either way it doesn't change my recommendation.
Now here's where it gets tricky for you, as we still don't know what amperage the circuit you are running off is. There are a few ways to figure this out. Usually your panel is labeled telling you the number and location each breaker controls. If you can't find this you can plug in a lamp and flip breakers off one at a time until you identify the circuit ( the light will go off once you find the right one. This will also allow you to identify the rest of the circuits in your home if you are unlucky and go through all of them. Keep in mind every time you flip one you will kill power to everything on that circuit; so hopefully no one is on life support.
Depending on what you find- If you have a 20 amp circuit and it's the only one in your miner space, you can safely run 2 S9's as is, or have an electrician replace it with a 30 amp circuit. Maybe you are lucky and find it's a 30 amp circuit there you can safely run 3 S9's. Rule of thumb is to only load a circuit to 80% of it's amps rating.
Whatever you decide to do will involve buying a PDU- you will size it to the circuit and voltage. This is important so pay close attention when looking for them, generally there are plenty available on ebay used. from here it is all about cord ends and making sure you identify your plugs both male and female ends. Make sure you read up on the PDU to see how many amps each socket will put out and how many each side of the PDU can put out. I say this as some PDU's only do say 5 amps for example at each socket or only do 12 total on a side so you need to space them out when you plug them in. Some of this will make more sense when you start to research them.
The PDU might come with it's own cord but you will likely have to order one to fit your socket and plug into the PDU so again properly identify them google is great for this.
The next cord you will likely have to buy will run from the PSU to the PDU, shouldn't be hard as you already can see the connection at the PSU, but when you order the PDU you will be able to see the sockets it has available. Using this you can order your other cables.
Good luck; hope what I said makes sense