Be careful, it is illegal to export an out of proof gun which your Clarke most certainly is!
Sending it to the US opens you up to prosecution.
In this case, I would hold fast and consider sleeving, it might be economic.
PLEASE don't scrap it nor deactivate it, it is a one way street in both cases and times change. What is un-economic now may not be so in the future.
And yes, deactivation must be done fully and a proof house cert obtained or it would still be considered an active firearm.