I worry about the locking her in bit, as already said...in case of fire.......unless of course there is a key or like our door a latch she can use to get out, but still means the doors locked from the outside??.......
but generally, no, YANBU, you know your child, your house & the area you live better than any one here.........
TBH, I do similar with my own DD & its only her, so no siblings to worry about, & shes the same age, extremely sensible, can use the phone, can get out, but no-one can get in, & the Supermarket is at the end of our road, so its never more that 20 minutes, usually less
I personally think in those circumstance, it can be a good thing, as it lets her know shes trusted & instills self confidence, & lets face it, it can be hard these days to allow Kids the freedom we had to grow & gain that kind of confidence, especially living in towns & cities.
though I can appreciate we all live in different situations, so it can't ever be a 1 rule fits all, we have to weigh it up based on our own circumstance.
Unless things have changed recently, it used to be, there is no actual law on this, only guidelines