Hi there, sorry I was offline by the time you had replied. The main issue here is going to be your length of service, because if you have been continuously employed at your place of work for less than 2 years then your employment rights will unfortunately be somewhat limited. Most importantly, you will not be protected against unfair dismissal or constructive dismissal. This means that your employer can dismiss you or force you to leave for more or less any reason, and without following a fair procedure, as long as their decision is not based on discriminatory grounds (i.e. because of gender, race, religion, age, a disability, sexual orientation, etc.) or because you were trying to assert any of your statutory rights (e.g. requesting maternity/paternity leave, etc.).
So you may have been offered a specific job and started that, only to find it no longer exists. Without the protection mentioned the employer can simply either terminate your employment without you being able to challenge it, or they could give you an ultimatum of taking another job and if you do not agree to it and are forced to leave – again without being able to challenge it.
So in the circumstances you could try and negotiate with them about what you see as an ideal outcome but if you cannot agree on anything, and the option is that you have to accept either of the positions, then you will have to consider choosing one or refusing to do so meaning you could get dismissed as a result or forced to resign. As mentioned unfortunately you would not be able to take this further afterwards because you simply do not have the minimum criteria to be able to bring in a claim for issues like this one.
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