Your account is a multi currency account which allows you to transfer, hold and trade any currency IBKR offers. For each currency, we hold specific bank accounts in different locations, where funds have to be sent to, depending on the currency.
As a customer of Interactive Brokers (U.K.) Limited ("IB UK") (as determined through your legal country of residence) IB UK acts as an introducing broker for IB-LLC which is our US entity. Depending on currency and product your assets will be held in different locations.
If you go through the IB UK customer agreement you can see under point 8 that "IB UK requires IB UK Customer to remit all funds related to their IB UK Transactions or to their IB UK Accounts, or otherwise tendered pursuant to the IB UK Customer Agreement, either: (a) directly to IB LLC or (b) to a designated financial institution with which IBKR LLC Maintains a customer account. Funds maintained by IBKR LLC are maintained in customer accounts that are held in the name of IBKR LLC and are located outside of the U.K".
You can find the Interactive Brokers (U.K.) Limited Customer Agreement in the Customer Agreements page of the IBKR website.
Unfortunately we are not able to give you any advice or statement to determine if you comply with the UK tax regulations in regards to being a citizen of your Country living in the UK and if the above is considered as funds being held offshore. We advise clients that they contact their local tax advisor to determine how to file taxes.