I think the previous replies directing you to resources for Received Pronunciation of British English are the most appropriate suggestions, but you should be aware that accents within the British Isles are very varied (there is no single British accent.)

Even if you become very familiar with the 'standard' English accent you would likely have difficulty with Scottish (especially Aberdonian!), Irish, Liverpudlian, etc. accents.

What grade are you hoping to get on the IELTS?

I had a student who wanted to become an official translator (Japanese to English). I recommended BBC radio programmes, from radio 4 and also Mark Kermode's Film Review from radio 5. Mr Kermode has a doctorate in Film Studies, and gets very excited about his subject, so he speaks fast, educated English in a South of England British accent (same as mine). The student said it was very effective.

Another student taught herself British English from the Archers (a BBC radio 4 drama about the everyday lives of farmers). She said it was very enjoyable, and I expect she learned a lot of vocabulary about cows ;-)