If anyone can do it in the UK then it will be the Tippmann tech at BZ
If you have to send it to the US then use the advice as above to be ready for TSA, there should be no other issues with it crossing borders and on return it should pass UK customs in regard to the VCRA or other legal importation issues
Include details of the warranty/service work when you send it, Tippmann should also include documentation on return
Keep a copy of any paperwork ready to send if it does get queried on return
(note usual customs VCRA queries are raised by the courier to you then they refer your response to customs as appropriate, eg I've imported via airsoft suppliers and been asked if its a RIF, it wasn't and the package had never been opened in transit as they accepted my declaration, (or possibly xrayed it), on other occasions packages have been inspected)
Make sure your email & phone details are documented when it's sent each way to potentially speed up queries)