That does sound like a brilliant idea in principle, however I think it would be far too costly. FTTPoD might be an option depending on the set up cost (which we currently have no idea about)
A shame because it could have been a good solution
2 businesses are going to be using the connection as a critical resource, and you can't afford the cost of a second line? I'm not sure that's the best thinking ... as your business ought to consider 2 lines for redundancy reasons *anyway*. If you aren't considering a second line, then you must make sure that you can swap to PAYG 4G mobile if the worst happens: this will be your livelihood, after all.
The indication for FTTPoD costs is that the total cost (after February) will be fairly similar to the total cost today. That implies a massive shift from the "monthly" cost to the "up front" portion.
As you have an estimate of around 15Mbps, then you are likely to be about 1.2km, or slightly more, from the cabinet. You are most likely going to be at least
that distance from an aggregation node. The old-system up-front cost for that distance would be around £3k+vat. If you assume that the difference between Openreach's old/new annual rental is going to shift to up-front, then you need to add 3 year's worth of ~£700pa ... or another £2k+vat. Total of £6k.
Given that the house already achieves 26Mbps, the seller is a little unlikely to accept a £6k reduction in the asking price based on "poor broadband"; even more so if Gigaclear are due to install locally. At which point, the £400-500pa for a second FTTC line might suddenly become palatable.