It looks like EE have decided to stop selling T-Mobile and Orange contracts online, instead focussing on them in store only from now on.
If you try to visit T-Mobile it will automatically redirect you  to the official EE store and at time of writing this, the Orange site does work but when going to the store it links you once again to EE.
The EE website says this
CAN I BUY ORANGE AND T-MOBILE?
Yes, Orange and T-Mobile plans and phones are still available to buy, just not through our online shops any more. If you’d like to join Orange or T-Mobile, you can still pop into an EE store, ring our EE sales team or check out our official eBay shop for the latest offers.
With EE being pushed as the major brand and pulling out of online sales on the older T-Mobile and Orange brands, how long until all future sales are 4G EE only you have to wonder?
Is this good news? We think so as it means more people will be going 4G and who doesn’t love faster speeds.
Source: EE