Normal Termination:
An employer must give you, as a minimum, a statutory notice period. An employer has the option, if both sides agree, to pay an employee money up front rather than have them on the premises during their work period. This payment in lieu of notice (or gardening leave) is typically invoked where an employer is concerned that confidential secrets/ customer lists etc. may be taken by the employee upon their termination. This, or the effect on company morale.
Gardening leave has no effect on the actual termination date of your employment. In fact, you are legally employed until the date that your notice period would have run out. This means that you can't take up another job in the interim.
Cutting to the actual issue here, as you are still technically an employee (notwithstanding the fact you have taken gerdening leave) you are entitled to have your holidays. As such, you are entitled to be recompensed for any unused holidays not taken at your termination date. Gardening leave and annual leave are entirely different.
Your options are then: tell them to shove their gardening leave, sit it out and get payment in lieu of unused holidays, or, take your holidays before your gardening leave period commences (ie. before the notice period commences) , or, demand that you be paid an amount for your holidays on top of the gardening leave offer.
Statutory Redundancy:
If you talking about a statutory redundancy, the position is very different. A statutory redundancy payment is calculated at two weeks payment for every year of reckonable service plus an extra week. The notice period required is dependant on the length of service. Unexpired holiday periods are irrelevant in the context of a statutory redundancy payment.
A payment in lieu of notice (gardening leave) means that you don;t have to work out your notice period, but will still get paid - generally upfront. As such, if you can, take your holidays before the notice period kicks in. If it has already kicked in, the issue of unused holidays is irrelevant. You are not entitled to any extra payment for unexpired holidays.
Obviously, before you think about any of this, check your contract carefully. Any terms and conditions relating to paid leave must be included in your contract under the Terms of Employment (Information) Act 1994. The terms of a contract may offer more protection to an employee, they can never set terms that are less than the statutory minimum.