Are you in London? My 1Gbps link gives 800-900Mbps in these speed tests, and I'm fine with that. It's a pitty that some of that is unused because I need to use a VPN.
When I lived in a small town, my 500Mbps would give me >400Mbps consistently.
Where are you in the UK that you can get 300 but not gigabit? AIUI, you’ve either got Openreach FTTC (caps out at 80mbps), Openreach FTTH (1gbps), Virgin (1gbps) or various alt nets (usually 1gbps)
It costs around 70 USD per month. Pretty expensive considered it’s not even gigabit speeds. But as others pointed out I may have better options to consider.