Nr,
I used to use UKOnline on an 0845 number a couple of years ago when I had Highway - simply because they were the only ISP I could find tht supported channel bonding with no sign up etc. I checked and they no longer support single or dual ISDN.
A quick trawl, I know you've already done this, came up with ;
gconnect.net
easynet.co.uk
giointernet.com
actionisp.co.uk
Can't say I've experience of any of them but actionisp looked as if they may have a package suitable for your needs. I didn't find any great performance increase when bonding channels but then I wasn't using a true router.
There's always satellite Broadband if your wallet's big enough ! Article currently on BBC website re. rural Broadband access ;
http://news.bbc.co.uk/1/hi/uk/2856321.stm
Good Luck