I think argos are traditional wet paint / stove enamel hence slower, multiple coats, multiple drying times, rubbing down etc = expensive. End result is nice but chips easily.
Powder is a quicker process, more economic with materials, covers more evils in a single coat and much tougher. If they’re using gloss polyester then there is no need for a clear coat. So hence it (should be) cheaper. Maybe living in industrial North has advantages 🙂 saying that, London fixed gear forum has pages and pages on powder coating with London prices similar to what I’ve suggested.
I’ve generally found powder coat price never has any bearing on quality – usually reflects how much they can be arsed doing your job.