in theory yes,in real life maybe not.
It depends on what components,the way baggage checks & case scans are now you'll likely get pulled at the airport if you don't declare.
This will happen on return to the uk not at the us airport coming back out.
What are we talking about exactly ???
You have a duty free limit of £390,after that import duty is 2.5% & that is 2.5% of the entire cost of any item over the limit not just the difference.
I would declare electricals/components personally that are not freestanding appliances that can be switched on at customs to be shown working - tablets ,phones,mp3 players etc are fine - as long as they're unboxed they cant prove they weren't with you on the way out & already your personal belongings.
https://www.gov.uk/duty-free-goods/arrivals-from-outside-the-eu
Not worth the risk of having a £500 gpu confiscated imo.