What's the day counter on Tescos website for?

How long it keeps. But as long as it's not meat or dairy produce, it'll be a "best before" not a "use by" date, and most stuff is usable long after its best before date. Those crumpets will be fine for at least a week, if you keep them in an airtight container or ziplock bag after opening. After that, they might be a bit stale, but still edible; warm them up nd put a bit of butter on, that'll soften them up nicely.