I suppose it depends how you define 'reasonable'. It doesn't have to mean 'cheap'. £10,000 is not reasonable for a skateboard but it's very reasonable for a new family car. Do we have any evidence that the line doesn't require £53,000 of work and plant?
As for the USO:
"1.12 BT and Kingston are required to provide a connection upon reasonable request and at uniform prices, irrespective of geographical location. This requirement is particularly valuable to customers in remote rural areas whom the market might otherwise not serve. Where installation of a new line costs £3,400 or less, BT sets a standard charge. Where installation will cost over £3,400, BT requires the customer to pay the excess costs (plus its standard connection charge). Ofcom has concluded that a threshold is a sensible approach and that the £3,400 level is reasonable. Ofcom is now proposing to consent to BT charging non-uniform prices above £3,400 and proposing to publish guidance which will suggest BT applies the standard charge when costs exceed £3,400 for particularly vulnerable customers."