If you are using Collinson for a Covid test at Stansted, I really wouldn't worry at all about missing your test appointment. As long as you've paid, and can show the booking reference and a passport that matches what they have on records, and they are still open (ie not midnight), then they will see you. Collinson's branch at St Pancras in central London were very flexible when I turned up a week later than booked (long story as to why this happened)