I bought my boyfriend an iPad Pro for his birthday. I am travelling from New York via Dublin to to Aberdeen, Scotland. Am I okay to take the unopened and sealed iPad in my carry on? Will I be asked to take it out of the box?
Security at your departure airport may ask you to take it out of the box, although it's unlikely: security do reserve the right to more closely inspect any electronic device.
Customs will not require you to remove it from the box unless they have suspicions that the box contains something other than an iPad. They may, however, require you to pay customs duties (in this case, VAT) on the iPad if it was purchased outside the UK and is being imported.
If you remove the iPad from the box, you will likely not be asked to pay VAT since customs have no real way to differentiate between a device you're bringing for yourself and intend to take home, or one you intend to leave in the UK. That said, you should still declare it, even if you are unlikely to be caught out.
The following is my understanding, IANAL this is not legal advice.
There are two sides to this, security and customs.
Security will open something if they can't figure out from the x-ray whether it is ok to go on the plane or not.
Customs will open something if they belive it may not be what it appears to be.
So both would be within their rights to open the box but in practice are unlikely to do so.
Afaict your ipad pro is over the VAT threshold, so assuming it was bought ouside the UK and your boyfriend lives in the EU then to be legal you need to go to the red customs channel, declare the item and pay the VAT (and possiblly customs duty).
If you were bringing the ipad for your own use on a visit to the UK and intended to take it back out with you when leaving then it would be "reasonable personal effects" and you would not need to declare it.
I'm not sure what the legal situation is regarding a gift where the recipiant is only in the UK on a temporary basis, such that the item will be re-exported but by a different person to the one who imported it.
AIUI being in or out of the box doesn't change the legal status of the item, but it does change how suspicious customs are likely to be if you explicitly or implicitly* claim it is part of your personal effects.
* Remember walking through the Green lane is a customs declaration even though you don't normally fill out any forms or talk to anyone.