Congrats on your PPR! Well, truth be told, you shoud have prepared and received your UK PCC ever since so you don't get boxed into a corner now. The guidelines were clear that you need PCC for every country you visted/resided over 6 months. In your case, it was 3 years in UK. What were you waiting for to get the PCC? Send the passports with Nigerian PCC first and hope they overlook UK. But in the meantime, also process the UK PCC.
If AVO writes back and ask for the UK PCC, tell them you have requested for it and request an extension of time to get it back from UK. If they don't request for it, keep your PCC for future reference, you will need it. All the best.