What happens if I do not give the required notice of termination of employment to end my contract?
This would be a breach of contract and the employer has the right to sue for damages in a civil court (i.e. for the cost of supplying cover to do your work or for the value of lost business), or to withhold payment for the period of notice you were expected to work. In reality, this does not happen very often, especially since it is usually very hard for the employer to point to any financial loss as a result of your early departure. However, your failure to work your full notice could legitimately be mentioned in any reference you request from your ex-employer.
Your employer is not allowed to withhold wages or holiday pay that have already been earned from workers who do not work their full notice unless the contract contains a clear written contract term that allows this.