Alternative offer of employment blocked

I work for a UK ltd company that is US owned. In February my sales role was made redundant. On 9th of March I was offered a similar role (6 month trial) for the US company that I accepted subject to a contract, visa and relocation package being in place and all were verbally agreed to at that time. I subsequently received a contract that was short on detail but did detail a 6 month trial and relocation package. They now want me to go to the US and work on a temporary visa. My opinion is they are asking me to work illegally (temporary visa is incompatible with a relocation) and I can’t take up the offer. We are now at a stand off as they don’t seem to want to force me to go or make me redundant (I am still in consultation) and I don’t want to go unless an agreeable contract is in place. Am I right in thinking they are unfairly dismissing me if they don’t offer a legal contract after I accepted their offer?

The fact that you no longer want to take up the offer on the new terms doesn't mean it's unfair dismissal, no. They can make an offer of alternative employment on whatever terms they like, and you are free to refuse that offer. As the new job is relocation, you will still be entitled to redundancy if you refuse it.

You accepted the offer subject to certain terms and conditions, you are not satisfied that those terms and conditions are being met in the contract you've been given, so you can refuse the offer.

Eleanor,

Really appreciate the reply.
It may be unusual but it's not that I want to turn the offer down, indeed I accepted the offer very early on and my acceptance is still on the table. The real sticking point between us is the time I can be in the US, If they had sorted a visa at the time of offer (which they put into writing the type of visa I would need if I accepted) I would be there now, but, to date they have failed to apply for this visa. They are now saying it is my fault I can't start (literally) tomorrow, but without a visa its not possible. Hence my feeling that they have made mistakes that have prevented me from starting the position and it is their actions that are dismissing me from the alternative job offer.
p.s. they have no problem about paying me my full redundancy.

It still wouldn't be unfair dismissal though, it just means you can instead take up redundancy as the alternative you are being offered isn't viable. If they offer an alternative and you are not happy with the terms being offered, perfectly justifiably by the sounds of things, you can just decline the offer, you can't force them to change the terms.