Thanks for the clarification. No offense intended about asking if you were sure it's really a bonded service, but just wanted to be sure, as some people aren't aware that just installing a second DSL connection and distributing traffic across multiple IP addresses isn't the same thing.
If there are only two pairs available at the pole (cottages at the end of a lane?), then for anything further BT will have to run new cabling from there back to wherever they can pick up more spare pairs. Depending upon the geography that might be quite a distance, and they may not be willing to do it, at least not at reasonable cost to you. It also depends upon current usage of pairs back to the exchange. Pairs can be shared for non-ADSL customers by using DACS, but every ADSL line needs a dedicated pair all the way back, so if there are simply no spare pairs in a feeder, it may not be possible without a lot of upgrading of external plant.
All you can do is ask, and see what they say.
P.S. My last home in England was also quite rural, about 5 miles from the exchange at the edge of its service area, so ADSL was limited to about 1 Mbps downlink at best. With deteriorating line plant some of my neighbors could manage not much more than half that, and there was such a shortage that I think just about every non-DSL customer in the neighborhood was on DACS, and in the village about a mile away they'd already installed a line concentrator as well. Last I heard, they were scheduled to get FTTC to the cab in the village, but where I lived would still be about 1.5 miles away from the cab.
I think the government's aim of 10 Mbps minimum or whatever it is for everyone within the next few years is something of a pipedream for many parts of rural Britain.
