Here is the opinion of www.homebuilding.co.uk on the subject:
'One example might be that somebody approached the owner of a piece of land, asking if they could buy it for use as an allotment and, as they were only going to use it for that purpose, that was reflected in the price they were prepared to pay. The owner may have wondered how, if in the future the land might benefit from planning permission, they could get their just share of the subsequent uplift in value. And one way would have been to retain a strip of land to the front, adjoining the Highway, with a gap that was sufficient only for pedestrian traffic.
Another common reason for a ransom strip is where the developer of an estate puts in all of the roads, sewers and infrastructure for an estate at the edge of the village. The fields beyond the estate are, at the time of the original development, outside the Settlement Boundary. But the developer might think that the time may come when those fields are brought within the definition of land that can be developed. ‘Why,’ they might think, ‘should the owners of that land get all of the benefit of the infrastructure we have paid for if their only means of access is via the roadway we have constructed?’ So, at the end of the cul-de-sac they might retain a strip of land between the carriageway and the boundary. The carriageway gets adopted and becomes Highways land but the ransom strip remains in the developer’s ownership, so that anybody wishing to develop the adjoining fields with access through the original estate has to negotiate with them.'
Thus the existence of the ransom strip might depress the value of the land sold and thus reduce any gain realised on disposal, thus minimising CGT exposure. Any fees received or the use of any ransom strip would form part of the owner's [ie your] income and such fees subject to Income Tax at your marginal rate.