DS (Year 6) and DD (Year 5) walked to school alone together this year. It is a journey similar to your DD's - about a 15 minute walk with the only "busy" road being the one outside the school itself where there is a crossing. They've been doing this for the last two years, so since DS was 9 and DD was 8. I wouldn't have allowed it if it had just been one of them at that age though, but in September DD (10) will walk by herself most mornings as DS is going into senior school.
If she is sensible I would maybe start letting her go into town but not just to "hang around", only if there is something specific she/you need to buy. Getting to the big town near us involves a train or bus ride so I haven't let DD or DS go in by themselves yet, although if there was a big group of sensible friends going I probably would. Again, with meeting up with friends at the park, it would depend on how many of them were there, how many I'd met etc. If there were only three or four, or I didn't know who any of them were, I wouldn't allow it.
I think if you are going to let her out by herself/with friends she should have a mobile phone so she can contact you for example in an emergency or if she is going to be later home than she said.
We got DD a phone for her 10th birthday. It's a fairly cheap one, I think the handset cost about £30 but it's pink so she LOVES it . She's on Tesco Mobile, on the Value tarrif. We chose this because it was cheap for texting which is mainly what we thought DD would want to do (most of her friends have phones), and also because on that tarrif you can't go on the internet or send picture messages, so she can't waste all her credit.
We have agreed to top up her phone £10 every 2 months, so she has to learn to budget; if she has used up all her credit after 6 weeks she doesn't get any more until the 2 months is up. I think it's important to have limits like this, to teach her that she can't just use up all her credit and then demand more whenever she likes.
I would ask your DD why she wants a mobile. If it's so she can keep in touch with you/her dad when she's with the other parent then fair enough, but if it's just because "All my friends have got one" then that's not a good enough reason imo.
BM x