How are you connecting the monitors to the computer? Are you using an interface?
I ask because if you're using an interface, you'll obviously connect through the 2xmono outputs from the interface. If you are just using a simple built-in sound card, then you'll likely need to buy speakers that connect via 3.5mm
If you need something that connects to 3.5mm, then you're largely reduced to fancy PC speakers. If you're connecting from an interface, then for my money, a set of KRK Rokit 5s would be perfect. That said, probably about £100 over budget, unless you find some used.
Si