The MX series might have different geometry, but I think you'll be hard pressed to find a fork manufacturer these days anywhere in the world who doesn't make their forks long enough to be able to be swapped in and out with suspension. Because it works both ways, the people who make these forks for Halfords won't just sell them to Halfords...I don't believe this is true for the specific bike in question, a Boardman Hybrid. The frame for their suspended hybrid (the MX series) is a different geometry.
Michael