Holidays are usually worked out pro rata fior the amount of the holiday year you will have worked. So if you work for 6 months you will be entitled to half. If you have not taken the allowance, discuss with your Company whether you can leave early or take days during your notice period. Should you already have taken more than the pro rata allowance again a discussion will need to take place to either agree for you to work a few days more or take a deduction from final pay.
Sorry but I don't understand the question- I don't understand the bit about "depending on when the start of your holidays was".
Your holiday day entitlement on leaving will depend on your annual holiday entitlement (prorated according to the proportion of the holiday year you have worked up to the leaing date) less any days holiday you have already taken.
Many firms expect you to take unused holidays before you go so there will be no holiday payn leaving