Sabbaticals are not governed by law, and so any arrangements will need to be contractually agreed between employer and employee. In an ideal world, the employer will have a defined policy which sets out expectations. However, many employers do not and when it comes to granting sabbaticals or career breaks, it is usually the employer that holds the power.
As there is no legal guidance, it means that (subject to any contractual policies) these breaks can be paid, or unpaid, and for such length of time as may be agreed between the parties. Many employers operating sabbatical systems do so as a reward for long service. Employees should generally expect to return to the same employment on the same terms but should always ensure this is clearly agreed before taking the break!
In most cases, the employment relationship will continue during the sabbatical, meaning that continuity of employment is maintained. This is relevant when it comes to redundancy calculations for example, or unfair dismissal rights. Whilst the relationship may continue, some of the terms may be varied or suspended such as salary and benefits. Holiday entitlement will continue to accrue even if the break is unpaid. Depending on the length of the break employers may require employees to take accruing holiday during their period of absence.