Quantum.uk service has security measure that you should enable, it is called a whitelist option. It is very simple to set up and manage.
When the whitelist option is turned on, you will be able to make withdrawals only to the addresses that are whitelisted. Please keep in mind, that if you add a new or delete all addresses from your whitelist, you will not be able to send funds for 24 hours.
To enable withdrawal addresses whitelist please login to your account and visit security section.
Choose cryptocurrency and enter the address and save it.
When the whitelist option is turned off, you will be able to make withdrawals to any address, without limits.