If you have children and you live comfortably above the poverty line, you need to pay £5 for yourself and £5 for each of your children. Zakat ul Fitr is calculated as £5 per person. If you have elderly parents and you are their caretaker, you must also pay for them.
If you have 5 people in your family including yourself, the amount to give is £25, if you have 10 people in your family including yourself, the amount to give is £50 and so on.
Zakat ul Fitr (Fitrana) is different to the standard Zakat because Zakat ul Fitr is only to be paid in Ramadan, before the Eid Salaah is prayed and can not be paid after this. If you do pay it after the Eid Salah is prayed, it is not counted as your Zakat ul Firt. Whereas your standard Zakat is calculated as 2.5% of your savings on a yearly basis according to a lunar calendar.