A company in administration basically has no money. You will be a preferential creditor in the administration and will rank towards the top of unsecured creditors. That means that any wages and holiday pay you are owed are paid by the administrator once the assets of the company are sold off and any loans like floating charges and mortgages are paid off, if there is anything left in the pot.
If there is no money left to pay you, the government runs a guarantee fund which pays you up to 8 weeks pay, capped at £310 per week (off the top of my head: may have gone up recently), and up to 6 weeks of holiday pay you are owed.
Your rights if your employer is insolvent : Directgov - Employment
The administrator will try to sell the company as a going concern, and if they do, your employment will be protected by the TUPE regulations.