Well firstly, medical condition or not, I don't think anyone who has gotten an offer from previous years should be allowed to immediately assume or think they're guaranteed an offer in any subsequent admissions round. Also, this advice is coming from a professor at Imperial, not UCL, that professor may very well be wrong about UCL wanting you back since he doesn't work there. Your chances for admissions are different every year and you are supposed to be judged relative to others in the same year and applying in the same application round. There is every chance that applicants were just too good that year or you got very unlucky.
However, I do think they should give you at least some leniency, if you are a home student, you should be eligible for contextual offers if you told them directly (i.e. not through personal statement), it does say on their website that any disabilities should be told to them directly. It is also important to take into account that this is during COVID to post COVID and a lot has changed since 2016, this year is also famous for being one of the most controversial, unexpected, weird and competitive application rounds in history whether you're in the US, the UK, Hong Kong, Germany or otherwise (especially in the US and especially for STEM subjects like yourself), I personally don't think just attempting one year of an undergrad degree is enough "intent".
Having said that, I do not think that this should discourage you from trying again if you are willing to, and I don't think it's something thats worth fussing over, UCL isn't the ONLY institution there is. Personally, I am not an admissions officer, but I can tell you that schools don't tend to be that mean, in fact, they want to widen participation whether it is because they can boast about diversity or create a multiplier effect to attract more talent as they see widening participation. You might be able to appeal if you do think they judged you on an unfair basis or biased basis, even tell them in person about your health problems, though I doubt the effectiveness, unfortunately institutions like UCL are sort of spoilt for choice when it comes to selecting high achieving students with great qualifications.