I've done a little looking around and figured out the following.

According to http://www.wheelchair-ramps.co.uk/ramp.html the gradient for a wheelchair accessible ramp must have a maximum individual flight of 10m and maximum gradients of 1:20 if longer than 5m, 1:15 if longer than 2m or 1:12 if shorter than 2m.

So in this case, assuming the height of the western street level is 2.5m above the subway level, we would need 50m of ramp. Assuming straight sections (flights), this would be broken up into at least 5 sections, because no single flight can be longer than 10m. The available space is insufficient for 10m lengths so there would be more than 5 sections or they could make it one long curvy ramp with level spots ever 10m. Basically, they may be able to make the entire subway into an accessible ramp. There would be no space for stairs.

I'm not sure that the WH Smith extension is the problem. The budget for replacing the subway with a single ramp would be very costly. Considering they're planning to spend £104K on re-painting, water proofing and recladding of the steps, the taxpayer would get bilked for crazy amounts if actual construction were involved.