do i declare subject im taking privately on ucas?

You must declare all qualifications on your UCAS application, but that leads me to ask, if it is not essential for the course you are applying for, why are you taking it.
For a predicted grade, you can ask any private tutor to email you school and ask for that grade to be added to their refence, or your private tutor can email each of your Unis individually.

Yes, you must declare it.
You add the exam centre you're completing it at as a part time place of study.
For a prediction, are you being tutored or entirely self-studying?

thanks and im self studying

im taking it because i feel like i’d be helpful for engineering ive spoken to a few people that didnt do it and they recommended taking it as they found it hard

What is the extra A level subject.
And is it actually stated as a required or preferred subject for any of the Unis you intend to apply to.
If it isnt, there really is no point in taking it. If its not mentioned on the specific course page, it is not going to make any difference to your application or your chances of an offer - the Uni will just ignore it.

its not required i want to do chem eng and theyve said that further maths or physics are preferred as 3rd subjects, im already doing physics so is there really no point? would it not strengthen my application??

Okay, in that case your ref just needs to comment that you are self studying and therefore can't provide a prediction for that subject.
Unis are well used to this scenario.

its not required i want to do chem eng and theyve said that further maths or physics are preferred as 3rd subjects, im already doing physics so is there really no point? would it not strengthen my application??

Its 'or' not 'and'. That would suggest that taking both is not going to give you any additional advantage. This is the sort of question worth asking at a Uni Open Day - or emailing the Admissions' team at that Uni and asking.
Be aware that any extra subject must be worth doing - if it takes up too much time / energy, or distracts you from the other subjects, and your other grades suffer, its not worth the risk Remember that AAA will always look better than ABBB, and that could cost you a Uni place on Results Day.

hi,
i want to take further maths privately even though my school offers ( its too late for me to pick it ) and im wondering how/if i need to declare im taking it on my ucas application?
and if i declare it, how will i get the predicted grade?
thank you