If you have access to a commercial phone line, then you should be able to connect your laptop using a calling card with a connection program. I use 'Laptop Link' and a World Telecom calling card; however, it doesn't always work that easily. Alternatively, you might be able to use a commercial line 'directly' and pay for your connection time if you have some repayment system in place and are honest enough to record your calls. We had such a system in place in a certain theatre of operations and you just logged your calling time and were billed accordingly. But it connected at about 17K and cost nearly £2 per minute!
The only reliable method seems to be to connect using GSM - if it's available. Not in the sun-washed Islas Malvinas, of course. But GSM is very slow and horribly expensive - and it will be harmful to your blood pressure when you find your Inbox full of spam e-mails.......
Most of the 'unlimited' deals in the UK are indeed tied to your domestic line and you need to have CLI enabled to use them.
I suspect that if you tried to link your laptop to a military line (even via your own calling card), Plod would find some reason to say that it's contrary to some paranoid security rule.
The Yanks have had detachment Hotmail and free Internet readily available for years; regrettably the pornographic filth which some seem to wallow in on the Internet has caused even the US Armed Forces to think again and now many wholly legitimate Internet sites are barred - I couldn't even access the London Stock Exchange website as the dumb Spam computer wouldn't allow access to it - 'speculative investment' it said; I guess that meant they thought it was a gambling system?
What we really need is a system where you buy a block of time from a morale and welfare provider, plug your laptop in and connect to your own ISP. Can it really be that hard? If it's possible to get live colour TV back from Pluto, surely this isn't beyond the wit of man?