Most brands pick their first UK manufacturer by reply speed. Fastest email wins the order. That is a sales metric, not a vetting metric — a factory that replies in twenty minutes and one that takes three days can both be excellent, or both be wrong for your product. This guide sets out the five compatibility checks to run before a single sample is requested: product match, MOQ alignment, technical capability, legal standing and communication quality. A fail on any one is a hard stop.