I'm starting to sound like I have shares in Admiral or something, but there's just so much misinformation...
With Admiral you choose your annual limit as you would normally (mines 10k) and it has no different impact than it would with a non-black box policy (there wasn't much difference in cost between 3k miles and 10k miles IIRC, we're talking tens of pounds).
You make it sound like they are sat there watching your every move. Lets be realistic, that just ain't happening, it will all be calculated by computers. If you're planning on robbing a bank and using your car as a getaway vehicle, then you might want to avoid a black box, but if you aren't committing crimes* on a daily basis or intend to lie when filling out your details, then there is very little reason why you shouldn't get a black box.
*Now, I wouldn't possibly break the speed limit, but my friend *cough* has a black box, drives in a... spirited... manner, not particularly worrying about adhering to speed limits, letting the car breathe while exiting roundabouts, and making sure to enjoy his car to the full when it's safe to do so, but regularly gets a gold / silver rating for his driving.
As I say, you are only going to get penalised if you are in the very bottom % of drivers. So, think of the worst driver you know, who drives it like they stole it and thinks every road is a rally stage - do you drive like them every single day? If so:
1) You shouldn't be driving
2) Don't get a black box
3) Make sure your job doesn't require a car cos eventually you are going to get disqualified from driving
Even if you score bronze on every journey, there's going to be plenty of worse drivers. You may not get a discount next year, but it probably won't go up either. It's a reward based system. The less of a risk you are, the more discount you will receive. If come renewal time they penalised people and increased their premiums, then everyone would just get insured with another company.
TL;DR - Stop being paranoid and do your research.