The value of a tax refund depends on your earnings and how much tax you paid in that period.
Whenever you apply for an income tax refund, you effectively reclaim the part of income tax that you have overpaid in the tax year in question (you are currently allowed to claim back overpaid tax going back 4 tax years).
In some cases, it is possible that all the income tax that you paid in the tax year when you left the UK could be due back to you; in other cases the refund value will only represent a portion of the tax you paid.