Depends on what you want to do with it. If it's just for riding around town on it might be alright. If it's going to be used for park or street riding you're better off replacing it with a BMX. Trials and BMX bikes are designed for different things, trials bikes aren't designed to take the same amount of abuse as a BMX. Even as just a starter bike I wouldn't expect a trials frame to last long ridden as a BMX.
£300 would get you an alright starter bike from a rider owned company like Wethepeople or Fit. You'll get a better bike for the money buying from them than traditional big box companies like Mongoose or GT.