whats wrong with the freewheel on that bike?
£300 for a full bike means they’ve skimped on the parts.
I had a Shimano freewheel on my singlespeed (cost £30) which lasted about 6 months. I replaced it with £80 worth of Whites Industries which is still going strong after zero maintenance about 3 years later…
If it has a flip flop hub then find a quiet street and play about with fixed and see how you get on with it. IME it’s personal preference more than anything.
Fixed has its benefits (low maintenance for e.g.) and it’s downsides (takes skill and alertness to ride in traffic, can have serious consequences if you forget to stop pedalling, the chain comes off when riding or losing your finger if you stick it anywhere near the chainring or cog when cleaning/doing maintenance)
Give it a go in a safe environment and see how you feel…