Is it possible to receive cash in foreign currency for work abroad?

Question

Individual entrepreneur

Hello, I will briefly describe the situation. I have an individual entrepreneur in Russia on the simplified tax system 6%. For some time, remotely from Russia, did some web development and administration work for a company in the UK. I have not received any payment yet at the moment, but some amount has accumulated.
I had the opportunity to visit the UK personally for the purpose of tourism, and as a result, the question arose: can I receive these funds in cash in pounds by concluding an invoice agreement with the customer at the place and will the tax authority of the Russian Federation find out about this receipt of funds? Is there any interaction between countries to inform each other about such cash transactions? This option suits the customer side, this invoice-agreement, most likely, will be shown for their tax as an expense. Thank you.