I have a euro account with hsbc and have been able to do sepa transfers from the exchange into my bank account.
Im sure you should be able to just transfer euros to your UK bank account using the IBAN account number and your bank will change it to UK GBP. Do check with your bank though to make sure this works.
Another option might be transferwire although i have not tried that myself.
ok i signed up as im interested in what your building
pricing model seems steep considering most exchanges charge 0.5-1% exchange fees. Bex.io fees are $99 a month + 1% of Exchanges. Does that mean 1% of trades?