If you get a SIM on Three now, you won't be roaming on O2 or Orange - but I really don't know if existing subscribers will continue to do so. Certainly those who roamed on O2 (2G) in the early days continued to do so when newer customers roamed on Orange. It can be set on a per-SIM basis. Perhaps I'll ask Three when I see them next.
As for using a mobile phone when driving, I am glad you've acknowledged it ISN'T illegal to talk on a phone when driving - as long as it's hands-free.
I don't personally regard it as being as dangerous as drink driving because if I am on the phone (which is very rare these days, thanks to SMS, IM and email) then whenever I need to concentrate I'll stop talking or say 'wait a second'. If I was drunk, I wouldn't have any way to temporarily sober up! I think it also depends on what your conversation matter is. Any call I am on will be as short as possible and the caller will be told I'm driving, so 99% of the time it will a case of me saying 'Let's catch up in 20 minutes' or something.
Of course, some people don't use common sense or still hold their mobile (this seems to be increasing possibly due to lack of enforcement) but that's no reason to ban the use of a phone in a car altogether.
Finally, it's common to have issues with using a phone when entering a tunnel from either end as the way a cellular network operates, there's an element of guesswork when someone is moving. As you move, the cell will hand off to where it expects you to be (and if it fails, will hand back). Depending on the position of the sites on either side, you might find that one way can maintain the signal and hand off successfully - and the other way it will fail and then have lost signal and have to hunt for sites.. by which time you're sitting with no signal. It's also why it can take time to re-register when you leave the tunnel even though if you were standing still by the tunnel entrance you'd possibly have maximum signal.
When I was working at What Mobile back in 2003, Orange was stating its intention to put sites by tunnels to offer coverage throughout (rather than using leaky feeders which I presume wasn't possible as it would have required permission to go onto railway property). Given there are still tunnels without any phone coverage, I have to assume Orange never succeeded in doing this!