My advisor signed a contract to hire me, but then external funding fell through. Should I force him to take me anyway?
Your offer is made by the University, not your advisor. Someone in admissions probably messed it up, but fwiw this is neither your, nor your advisor's problem. As soon as University gave you an offer, and you met the conditions, it's legally binding.
Many other answers are written under assumption that PhD is an employment and hence &quot;probationary period&quot; or &quot;redundancy&quot; can be applied. This is not correct. In the UK, PhD is considered &quot;training&quot;, not employment. The University's offer for you is their promise to provide you with a particular service. Even if the industry partner goes bust (which is not impossible), the University is still under obligation to fulfil their side of the deal. If the University absolutely can not serve you (e.g. the only qualified professor has just left, the lab building collapsed, their license is revoken), they must find another suitable program for you and pay your tuition fees and relocation.
These obligations are not merely ethical, but can be and often are enforced. The relevant regulator in the UK is  the Office for Students. Obviously, I am not a lawyer,  so I can't give you any legally reliable advice. But I believe you are right and OfS will be able to support you if the University disagrees with your arguments in this case.