Looking at the properties concerned, and knowing people in the commercial property valuation business, I reckon if the three units were on the open market they‘d sell for a total of around £10m-£15m. However, if you wanted to persuade the owner to sell them, ie not on the open market (as would be the case here), you’d expect to pay a premium of around 20% on that, plus the sellers costs. The mere fact that the project has been investigated at a cursory level is enough to make the owner a ‘reluctant’ seller (as well he ir she might be, fir an extra few million). Add in your own purchasing costs, and it will be nudging £20m.
Then there will be the issue of the Order required under the Transport and Works Act, regardless of whether the land is already in railway possession or notm- assuming that once you own the property that you want to build a railway. Even for something simple like this, it would be several million, and that assumes you don‘t need similar consent for any other part if the line (which I suspect would be required).
Then there‘s the demolition costs, site remediation, and so on. All told, I’d be surprised if just getting a clean canvas to start building the 1/4 mile of new railway would be less than £30m.