Am I an EU/ UK national?

I was born in Nigeria and have lived in the UK (London for 21 years). I hold a Nigerian pport but I have an indefinite stay stamp in it and I also have a home office letter confirming that I have indefinite stay in the UK but I don't have a British pport. I've been told I'm a British citizen but I'm not sure if I'm a UK/ EU national. Can anyone help me to confirm my status?