Take a look at this
the relevant sections are 82 to 85 and 95 to 98
This may also give you assistance Sections 15 16 18 35 and 36 are the relevant ones.
There appears to be no reason under UK law you cannot import the hammer from the US as your non commercial import for personal use should have the 'Domestic Authority' aspects covered by your license. The ITAR regulations may mean you have to jump through a number of legislative hoops (in terms of paperwork and a small processing cost) if the Feds have designated it under ITAR but it is unlikely to breach the $100 limit if it not specifically covered by ITAR.
Hope this helps.