This isn't a problem I've come across much in this area (rural east Norfolk). That said, my own house does share a single-phase branch from the nearest pole with the house next door, looped from the older-style individual overhead cables. But those cables appear to be 25 sq. mm. (or possibly the earlier 19/.052 size) and there's only one of us with electric heating loads (myself).
I suspect you may have a hard time getting the PoCo to foot the whole bill, but £2000 seems a bit steep for just running another overhead service drop.
Is the pole serving these two houses already run with 3-phase 4-wire, or is it just single-phase from a xfmr serving just these properties? If the latter, then maybe they're expecting a contribution to upgrade the xfmr.
