Technically you're in breach of contract. In practice, three things commonly happen: the employer doesn't pay you for any unworked days, the reference may become neutral or negative, and there may be a note on file. Damages claims against individual ex-employees are very rare because they're rarely cost-effective. Walking is uncommon but legally manageable; it's the relationship cost that's harder.
Notice periods are mutual contractual obligations. If you don't work your notice, you've breached the contract. The employer's remedies for breach are: refuse to pay you for unworked time, withhold cooperation on references, sue you for damages caused by your premature departure, or include the breach in internal records. The first three are situational; the fourth is automatic in most HR systems.
Salary withholding is the most certain consequence. Your salary is paid for days you actually worked (or were on approved leave). Days you didn't work because you walked out aren't payable, and the employer won't include them in the final pay packet. If you worked half of a four-week notice, you'll get two weeks of salary plus accrued holiday, not the full four. Holiday and bonus accrual stop when you stop turning up.
References get more complex. Many UK employers now give 'factual references' only (start date, end date, role title) regardless of the circumstances of departure. That's the default safe position for them, because subjective references carry defamation risk. A factual reference doesn't reveal the breach. But some employers will tell a future employer if asked specifically about the manner of departure, and that's where reputation damage can happen.
Damages claims against individual ex-employees for failing to work notice are very uncommon in the UK. The reasons are practical. Damages are limited to actual loss caused by the early departure, which is usually modest (the cost of recruiting and onboarding a temporary replacement, minus the salary the employer didn't have to pay). Litigation is expensive. The optics are poor. Most employers don't bother. Senior or specialist roles where the disruption is genuinely high are the exceptions.
Regulated industries are different. Financial services firms, regulated professions, and certain public-sector roles have formal vetting between employers that catches contractual breaches. A future regulated employer will ask the previous regulated employer specific questions and will receive specific answers. A notice walkout in financial services can damage employability in a way it wouldn't in most other industries.
There are circumstances where walking is the right call despite the consequences. Constructive dismissal (a fundamental breach of contract by the employer that releases you from your obligations) lets you leave without working the notice and potentially claim. The bar is high; ordinary unhappiness doesn't get there. Harassment, unsafe working conditions, sustained unpaid wages, or a major unilateral change to your role can sometimes qualify. Take advice before relying on this.
PILON arrangements remove the walkout question entirely. If the employer has a PILON clause and chooses to use it, you stop being employed immediately and they pay you for the unworked notice. This is voluntary on their side unless the contract requires it on a particular trigger. Asking for PILON is sometimes worth doing if working the notice isn't feasible and a clean financial settlement suits both sides.
Practical recommendation: don't walk on impulse. If you genuinely can't or won't work the notice, talk to your manager first and try to negotiate a shorter notice or PILON. Get any agreed shorter notice in writing before stopping work. If you have to walk anyway (and sometimes that's the right call), expect to lose pay for unworked days and assume the reference will be neutral at best.
General information about UK employment law. For your specific situation, contact ACAS or an employment-law solicitor.