While I agree with this ... giving your old UCI number will almost certainly not hurt (unless you got it wrong).Just don't put any number or UCI there but the system will still find the same number fir you when you apply....The system will identify your date of birth and name give back the old UCI number....!!
AND: if you have a very common name, have changed your name, or a few other uncommon scenarios - it MIGHT help or at least reduce the chances of an inadvertent mismatch.
So mostly I'd include it if you know it.