It is unlikely that the title forms part of the terms and conditions of your employment. The key thing is, is your employment actually changing your terms - the hours, the the location or most importantly your salary? If they are not, then they can do that. It is not likely to be a condition of your employment that you manage 15 people, for example (as a cell leader might). So if the employer wants to employ you as 'Manager in charge of Nothing At All" at the same salary etc. you cannot do much about it (except resign).
Hammerman presents a slightly inaccurate picture of this in my view. He could insist that his job is being made redundant, and force the employer to declare it as such. However the statutory redundancy scales are so low that it doesn't gain him much. If the employer refuses to declare it as a redundancy situation, he could resign and claim constructive dismissal through an Employment Tribunal. What he is correct about is that an employee does not have a right to continuity of employment for life.