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Thanks for your question - I am Sam and I am one of the UK tax experts.
You should ensure that you have invoices that you raise to the offshore company detailing the money earned - which then will tie in with the amount that arrives in the bank account.
You then at the end of each accounting year complete a Corporation tax return with figures and accounts - and HMRC only need the bank statements and other relevant receipts/invoices if they choose to look further into your accounts. But you do have to retain this documentation for 6 years
BUT if this si to be your only client then you may have to be treated as an employee depending on the terms on which you are engaged (which will mean operating a simple PAYE scheme on your money have have tax and National Insurance deducted from it and paid over to HMRC monthly - so do ensure you are not in breach of IR35 legislation with this.
Let me know if I can be of any further assistance