If you want to keep the UK one, the PRC passport will be revoked instantly. I don't know what would happen if you wanted to keep the PRC one, because after going through all the effort to naturalise who wants to just give that up?** I guess they might hold on to your PRC passport pending your official renunciation of UK citizenship (though IIRC you have one opportunity to restore UK citizenship after renunciation).

Anyway it's one rule for some and another rule for others, I know someone who the PRC quasi-officially permits to hold PRC and Canadian citizenship but the less said about it the better.

**Well I heard of a naturalized American who didn't realise he needed to file US taxes regardless of his residence, but he had already renounced his PRC citizenship and wasn't allowed to get it back even if he gave up his US citizenship.