Repatriate money from NRE account to my UK account

I have an NRE account in India and a UK bank account here as I am a British Citizen and OCI.
As Interest rates in Indian banks are far better than in UK, I plan to have Fixed Deposit in NRE account from the money I transferred from UK account(taxes paid in UK) to my NRE account.
Once the FD has matured, Over a period of time I want to get my money back to UK.
I understand all interests accumulated will be Taxed here in UK and I would want to pay the taxes incurred with Interest, but if i get the money back into UK account in small installments, how do i breakdown the amount of the interest+amount transferred in these circumstances so that I pay taxes only for the Interest amount.