What usually happens with bursaries is not that you ask for an amount of money but that the schools assesses your financial situation against set criteria and then makes an offer - it is up to you to then decide if you can afford the difference between what they offer and the full fees.
The sort of things that look at are:
income
sometimes assets eg second properties
savings
They then make allowances for eg:
Other children
pension contributions (but not if excessive)
mortgage costs
Here is some info re bursaries from a school in BIrmingham - they all vary but this is an example of how they view the process
https://www.kehs.org.uk/uploads/Assiste ... mation.pdfhttps://www.kehs.org.uk/images/Admissio ... -19-20.pdf (PS I think this is a very generous scheme)