Thanks for your question - I am Sam and I am one of the Uk tax experts here on Just Answer.
Is the cash coming in from the EU or outside of it?
Anything over the the equivalent of 10,000 Euros (£8430) must be declared and you must complete form C9011 making a full declaration of the money
The link I have provided also gives you the online form C9011 for you to complete and print off
Do let me know if I can assist further