External company visit - my employer refused my trip request

I work at a public university and was invited to a conference hosted by an external company who is trying to sell its educational products to us as a subscription. The costs are all paid by the company, and they need two people to attend from the admissions department and someone in international relations. I work for international relations and comms.

However my boss declined my request, saying that someone with higher authority should go there instead, and then eventuallt decided that no one can go to represent our institution at all since its not a priority event.

The event requires a visa to the UK for me to go and I was very looking forward to going since I scored the project. The external company was kind enough to send me a formal invitation for my visa but I did not disclose this. It was done just in case I can make it. I communicated to him and my other boss very professionally more than once that I want to go and that this will help our university be more visible but he was very avoidant of the conversation and was not supportive - said sorry that he disappointed me very coldly.

Would it be crossing the line if I apply to the visa using the invitation and go privately? And take days off? Or would this be not acceptable because im representing the university? The (UK) visa needs proof that my employer knows I am going for business, but I was thinking to just go as a tourist....

Was my boss right about this? Is this normal or was he just trying to prevent me from going?