Soldato
Not sure how anyone could recommend passive speakers on your budget? You'd need to add an amp, and that blows us way past £100 for anything good.
Maybe a competent 2.0 active setup. Uncomplicated and adaquate. Something like the Edifier R1600t iiis. If this is for gaming and multimedia and spotify you're more or less covered. If you want more, you'll simply need to up the budget.
It can be done, it just requires a bit of work hunting around for secondhand gear, I bought a Denon PMA-510AE stereo amplifier for £60 and a pair of Morduant Short MS902,s for £40, exactly £100, and once you have that stereo amplifier your sorted for future speaker upgrades, it's not something I'd recommend for everyone but it's definitely worth it if you do it.