Thanks for all the replies. So it looks like the options are:
a) get FTTP installed at some open-ended and probably high cost. We're about five poles away from the new Fibre cabinet, but access looks otherwise straightforward
b) simply get a new line installed and hope that this would be routed to the existing telephony cabinet about four metres away from the fibre cab (i.e. 200 metres away) and thus be capable of FTTC-broadband upgrade
c) invoke the "open access to poles" I've read about somewhere and lay my own fibre to the general are of the cabinet and/or do a deal with the farmer whose field is in-between our office and the cabinet to put my own small poles up around the edge of the field (can't see BT allowing this sort of pragmatism, but it's worth putting it out there)
d) maybe do a deal with a neighbour that isn't EO (which I gather is most of them). Risky in the long term, obviously, if they move.
Re the comment about the difficulty of cutting bundled EO connections, would it make any difference if I could show that we were the /only/ EO connection? I recall a BT engineer a few years ago who came out to fix a line fault implying that that was the situation.
A new line would probably use one of the existing pairs of the line that you already have so you would still be in the same situation.
Basically your only viable option is to wait until there is a cabinet installed out side the exchange and your EO line is moved onto the new cabinet.
EDIT: .... or consider some other means of getting your broadband such as satellite broadband.
A cabinet outside the exchange is not going to help, as its going to be the same distance from you.
If that new line was to our new office - an entirely separate large building from the house - would the risk of EO line re-use be reduced? Given that there is a conventional telephony cabinet nearby would that be the first choice, or - as you say - would/could BT re-use an existing pair?
openreach will use existing pairs wherever possible rather than a new line