Originally Posted by
Skatering
The topic of multiple citizenships during UK passport renewal comes up on the British Expats forum a lot.
The consensus on there is "Don't send it. It's none of their business." YMMV.
Yes some countries don't allow you to hold dual passports with certain other countries so it might not be a good idea to declare the fact you do without checking first. No idea how strict the UK is on this but here in Argentina it can cause a problem if you spill the beans.