No prob. Just about to have some breakfast (!)
Don't switch to unlimited at 9.99 unless you plan to stay for another year. I should have mentioned it earlier but it had been a long night on the net and I was close to bedtime...
I know that you're paying a pretty high cost (but that's 'cos you chose an iPhone, rather than a 13/16/18 quid deal with some other phone, and the One Plan starts at 25 pounds anyway... don't think it is that high, my ex-neighbour on O2 with an iPhone from early 2012 was paying 45 to 50 before she dialled any chargeable calls like directory enquiries {hellish expensive from a mobile and I'd always try the BT website first, or a search engine for a business!})
Also there's the issue of insurance. If using a phone costing 50-150 quid, it's cheap enough to buy a replacement rather than be hit a fiver or more a month for 2 years, whereas a top end phone is more attractive for theft and costs far more to replace, another reason for some mobile contracts being far more costly than others. I've 'cheap and cheerful' phones (lots of them though!)
I know I would not expect mobile broadband (via a phone) to work well enough as a replacement for using a landline. It will mean needing to keep your phone on charge a lot more (I suspect) because using it for tethering it is transmitting a mobile signal as well as a local wi-fi signal for your laptop (and perhaps a bluetooth signal if you are using that). I 'shared' my USB dongle using a desktop (powered) unit from Three so I could access it from in the house, and while that was OK for a short YouTube clip, I know it would have been quite hot (and possibly have paused with 'buffering' a few times if I had tried to watch something using iPlayer. I could probably have downloaded OK, but it wasn't an unlimited a/c... 'generous' at 15 GB {seeing as it was only 8 quid a month, and on many networks it costs a tenner for 1 or 2 GB and they don't even offer 15 GB} but I used that and 60+++ GB via PlusNet (except when moving home and then the mobile facility was ideal before the phone line was set up with Primus and broadband reinstated from PlusNet).
Admittedly I typically download a lot more (20-30 GB then, 50-60 GB a week now) but my sister was without any signal from Three for a couple of days (Friday, Saturday, I think) so no mobile, no internet, if she had been using only Three. Sister is down near Brighton, so hardly "in the middle of nowhere" :)