Its going to be used outside to house some pixel leds. I'm going to cut a length into 50mm sections.
Ok, I think I have a full picture now. Don't think me rude, I'm trying to help you here. In the future it would serve you better to lay out specifically what you intend to accomplish and specifically how in your OP. TBH, It's been like pulling teeth to put this puzzle together.
I'm no expert on LED's but I do know they require a current to work. And if you're like the rest of us that means standard household current. Under the right circumstances it can, and has been, lethal.
It's a really bad idea to DIY an electrical project that you're going to bury in the ground, most especially if you have little to no understanding of electricity and construction techniques, using a metal conduit! (Please re-read that sentence.)
That said, I do think you can DIY this with a little research. The entire length of the run must be in a conduit, preferable PVC, or with an outdoor rated line. In addition the LED's must be rated for outdoor use. And the points at which they are exposed must be properly sealed. Anything short of that and you run the risk of a short circuit that may cause serious harm or death.
I'm American, I have no idea what the electrical codes are in the UK. But, I do know that they aren't significantly different from ours. Electricity is a well understood technology today. Best practices have long been established.