@Rawns
Not a hard job to be quite honest.
As Los said, skirting boards are the way, it just depends on the thickness. I've faced this situation, and here's what i did.
Ripped off the skirting boards, checked the thickness of the board against the cat5, slowly, cut out a trench within the board. This can be done with a chisel or if you don't have one, a straight screwdriver (would not recommend). Cat5 needs to be either LIGHTLY tacked or taped onto the board and with a steady hand + ear put back against the wall. DO NOT push the board too fast, you need to hear for a slight tkkk sound (meaning the cat5 is touching the wall). If you've done it right, it should sit perfectly and you will not be able to tell the difference. If i'm quite honest, this will take you in excess of 2 days, with a lot of patience required.
Alternative method done by a family member + me helping him. Behind the plaster, but that's if you can't find a solution any other way.
Got loads of other ideas, but this is the simplest. Holla if you'd like more advice.
Sunny
Edit: You could also cut out a trench within the wall, CAREFULLY, DO NOT HIT ANY ELECTRIC CABLES! and line it up along there. Skirting board right over and jobs a gooden. That'll take about a day if you're quick and accurate.
It's an easy job, stop stressing:)