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Maybe they changed the rules since April last year but the current site still says you need to show them "any other passports". It's like, line 2 of 2 in their requirements.
https://www.gov.uk/renew-adult-passport/renew
"To renew your passport you’ll need:
your old passport
any valid passports you have from a different country - send either a colour photocopy of every page (including blank ones) or the physical passport"
I caused much drama when going back to Oz via Bangkok, having entered the country on my UK and trying to leave on my Aussie :)
Renewing UK passports with dual is a cunt, because the scum UK want both. So if you travel a lot, like moi, you're basically forced to use their double price fast service just so some prick in an office can photocopy your other passport. They won't accept "user provided" photocopies, the wankers.