"You will not need your current passport for applying British Passport as you will get a Nationality Certificate and they will use that certificate to issue you British passport. When you apply for nationalization they will sort out everything and will return your passports without putting anything in it. Once your application for nationalization is approved and Home Office has issued you the certificate, they wash their hands off your native country?s passport. Don?t worry if you are applying for nationalization in January"
The Passport website states that you are required to submit, with your application, your passport with which you entered the UK. If you have more than one, because for example you changed your surname, it would probably be best to submit both. They should be returned to you if your home country supports dual citizenship.