Why tax on foreign income as it is not considered to get home mortgage by any leading banks of UK

Once you get Residency card of UK, your worldwide income becomes taxable and you need to pay tax to HMRC UK either arrising or on remittance basis. But for lending home mortgage also for first time buyer who has just got this residency of UK, his/her worldwide income is not considered to get home mortgage also by all the leading nationazed banks of UK. Hence HMRC need to remove the income tax on worldwide income and need to keep tax only on local income only.

Why is this important?

Coin has two sides. If you do not consider worldwide income for a first time buyer of home mortgage, then how come you can tax him / her on his / her foreign income?