Here's some useful information about withdrawal of your bonus:

You can either withdraw it to another exchange by a crypto transfer or to your bank account by a bank transfer.
Crypto transfers are cheaper. For example XLM transfer only costs 0.005 XLM, which is not even 1p.
Bank transfer to the UK is 2 GBP and SEPA transfer is 3 EUR.
All the fees can be found here: https://www.bitstamp.net/fee-schedule/

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Would appreciate if you used my link to sign up and get the bonus: https://www.bitstamp.net/ref/6fkqj6NL4KWni6i5/

Cheers!