I'd be in breach of my contract, and (in theory) my employer could sue me. 3 month notice periods are very common for senior people in the UK so employers work with it. It also benefits the employer because it means they're less likely to have an unfilled role - if I resign they have 3 months to replace me. Less notice means they have a serious problem after I leave but before the next person starts.

3 months isn't even the longest I've known. I worked with some people a few years ago who were C-level execs in a manufacturing business who had to give 6 months notice.