Not exactly same experience as you asked, but I know that when Home Office issue you ILR they also provide you a letter stating that you have ILR. Even if ILR is not stamped in your passport, that letter will work in place of actual stamp. You did confuse me that you will not have Right of Abode stamp in your passport. I answered it partly if you have the letter. If your passport is valid and you have ILR in thsi country then burden is on you to prove it. Immigration officers are usually flexible if you have other papers and you can prove it.
Many countries will revoke your nationality if you get another nationality, which can come into play meaning that your passport may not be valid. This area is bit tricky but can cause problem when you return.
In order for you to Enter UK you must provide some type of Right to ABode proof. This you can do with many other document then just British Passport such as letter or the Naturalization certificate.
You must have valid passport from another country as you won't be able travel to UK without it.