Many thanks for your patience. Employers have the right to choose who they employ and can make such decisions based on a wide range of factors. There could be a number of reasons why one candidate is chosen in preference to others or why someone is not given a job, even if they are generally considered to be the best or most suitable candidate. It is generally lawful for the employer to use whatever factors they feel are relevant and appropriate in the circumstances to come to that decision.
So even if the successful applicant is not immediately available to start the position, that does not mean that choosing them has broken any laws. This is a business decision for the employer to make and if they are happy to wait for them to return from secondment, even if others are ready to start immediately, then they are free to make such a decision.
The only requirement in law is that the employer’s decision is not based on discriminatory grounds. That means that it should not base its decision on factors relating to gender, race, religion, age, disability, sexual orientation, etc. If its decision is based on any of these, there will be a potential case of discrimination and the affected person can potentially take this further. However, in the absence of any discriminatory reasons, the employer will rarely be acting unlawfully and will have the general power to be selective over whom it employs, even if it this generally appears to be unfair.
I'm sorry if this is not necessarily the answer you were hoping for, however I do have a duty to be honest and explain the law as it actually stands. This does mean delivering bad news from time to time. I hope you understand and would be happy to provide any further clarification if needed. If you are still satisfied with the level of service you have received I would be grateful if you could please take a second to leave a positive rating by selecting 3, 4 or 5 starts at the top of the page. Thank you