Is it illegal to not serve out notice period at work? (Eng)

I am employed (15 years; same company, England) and my employer is committing fraud. Once i uncovered this, i whistleblew internally and then externally to a specific financial body. I have not heard more on the external investigation.

The internal investigation was carried out and the manager in question was reprimanded, temporarily removed from duties and made to undergo training. He has since been reinstated in full. But meanwhile I have been demoted to a very junior position with no access to financial information. Colleagues have been instructed not to discuss any financial information with me. I believe the fraud is still ongoing but since i now no longer have access to financial data i can’t prove anything. My day job now is mostly admin; previously i was a senior manager dealing with financial data.

Meanwhile the manager in question, and in fact all senior leaders, are making my life very difficult. They have set me unrealistic performance targets which i cannot meet and are now going down the path of writing me up for poor performance. They are also constantly asking for justification of my work and I have received a written warning for “my tone and behaviour” which never happened. I believe they will fire me in the next few months so I have started looking for alternative employment. I do not want to fight them anymore and my view of the company is beyond damaged so there is no way i can stay working here. I don’t want to involve HR or fight to stay, i honestly just want to leave.

I have had a few successful interviews, but they have been put off by my notice period (3 months) & withdrawn the offer once i mentioned this.

My question: if i find a new job this week, can I walk out of this current job without serving my notice period? What are the legal consequences for me?