My pumps and I have travelled many times through airports.
WE have never been through Stanstead and I haven't had a T-Slim.
However, the usual advice is that pumps should NOT go through the x-ray.
You do not have to disconnect it to go through the metal detector archway but if "invited" into the body scanner (the one where you have to put your hand above your head), you need to tell security about your pump. They may request you disconnect it but as patch pumps are not possible to disconnect, they (and you) may prefer to be manually "wanded". They will then swab your pump.
All of my pumps have had the option to request a "holiday loaner" pump from the manufacturer so you have a spare whilst away.
I don't know if T-Slim have this option but it is worth asking. However, if you do have a spare, do NOT put it through the x-ray