There is no single average that fits every home, because a loft conversion can mean anything from adding rooflights to an already-tall roof, to building a full rear dormer with a new staircase and ensuite. The type of conversion is the biggest factor in the final cost.
A straightforward rooflight (Velux) conversion, where the existing roof structure has enough headroom, is the most affordable option because it avoids altering the roofline. A dormer conversion, which adds headroom and floor space by building out from the roof, costs more. A hip-to-gable or mansard conversion, which reshapes the roof, is the most involved and the most expensive.
Other things that move the price include the new staircase, the level of insulation needed, whether you are adding a bathroom, the quality of finishes and fittings, and access to the site. Professional fees for drawings, structural calculations and building control also form part of the total.
Rather than quote a figure that could be misleading, we prefer to visit, measure up, and give you a written quote based on your actual roof and what you want from the space. That way the number you get is one you can rely on.