Eight is a hard year. Both my older girls were huge pains at eight. It's difficult for them, they're not little girls any more but they're so far from being teenagers. They're allowed to do some stuff, but they want to do more. It's natural for them to try to test the boundaries.
And that's what she's doing. Testing herself and you. And it sounds to me, like she's getting some mixed messages from you. She's old enough to ride to school by herself, but not old enough to ride around the block? That's a natural boundary to push.
I honestly don't think the problem is how you can punish her more, but it's actually how can you punish her less. You have grounded her for SIX weeks for riding the bike around the block-that just doesn't fit the crime. You grounded her for four months over the money thing. Not only is it an excessive punishment, but you've set yourself up here-what more can you do? What I mean is-what are you going to do a couple of years down the road when she really does something bad-smoking, sneaking out with boys, getting paralytic? Ground her for life? Lock her in her room?
No wonder she's rebelling. And lying. She probably thinks that if you find out these things you WILL send her off to live in Cumbria. Which is probably her worst fear. Believe me, even if you think you've never said that in front of her, if you've ever said it outloud she's heard you and she believes you.
I think you need to take a step back. First of all, she should be allowed to have friends over. If she's sneaking them in, maybe she's afraid to ask. You might need to negotiate who/when/how many. But then you'd both have a definate idea of what was going to happen. Same thing for the bike riding-maybe negotate a different route that's a little further but safer. Get your own bike out and go with her-that way you'll have a good idea of how safe she really is.
The money thing is a bit worrying to me-I think you need to look seriously at why she was doing it. Is she being bullied? Is she trying to make friends? Does she really not understand the value of things?
And finally, while I agree with taking away things like the tv, computer and playstation, I'd really have to draw the line at a comfort toy. That may seem like her last friend in the world and taking it away would just be mean. She is just a little girl-eight is not very old at all. What she wants and needs is to be kept safe and loved and be reassured that her little life is going to keep going along on an even keel.