Unfortunately, laptops take a lot more of a beating than desktops, and a lot of it can be down to how you treat it (just in general, not saying that you haven't personally treated yours good).
Opening the screen one handed instead of gently with two, and putting things on top of the laptop when it's closed such as paperwork and books etc will all go towards deterioration of the screen and its hinges.
The mouse pad can deteriorate faster if you press hard down on it, or of course get annoyed and bang on it. I always buy a seperate USB mouse and use that instead of the trackpad on the laptop itself. Much easier to use and doesn't risk damage to the laptop.
The battery and charging port can all deteriorate faster if you always let the laptop overcharge, overheat and/or don't watch when plugging it in. eg if you just try to wiggle the charger into the port without watching it go straight in, then you're at risk of putting extra pressure on the charging port that's soldered to the motherboard.
If it starts running slow, then it probably needs resetting back to factory settings and updating to the latest Windows Updates, or at least having a good system clean of the cache, registry and any unused software.
£600 sadly is not a lot of money for a laptop these days, or even for one 3 years ago. You would be looking at double the price tag for that.