Credit Union

If we save 10,000 in the credit union for a deposit, can we borrow 10k on top of that? And if so will the banks know about it?

If you decide to pay back by Direct Debit , The bank in which you hold current account will know about it.

Depends on the credit union, some are listed on the ICB.

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Some are not. If listed on the ICB then the "banks will know about it".

Your deposit should be from savings or a gift. If there is a large cash deposit into your current account, the bank will question where it came from. Adding to what allthedoyles has said, if you repay by direct debit then these repayments will also show up on your current account and the bank will also question this.