Can an employer give my secondment to another employee due to their secondment no longer being available?

I was offered a seconded position on the back of an interview and I accepted this position, another employee was also successful and scored higher than me in the interview, they were given a choice of 2 positions and accepted theirs I therefore was offered the other position which I accepted, however a restructure meant their position was no longer available, my position was still required to be seconded and I had been given my official hr welcome letter to my position with start date and uplift salary. The business has now said I can't have my seconded position because the employee whose position has been affected is being placed In to my secondment as they scored higher. My frustration is that the employee did not initially accept the position given to me they accepted the position that is now no longer available, where do I stand in this situation is my employer in the right?