Vertigo - there is a ferry from the Hook of Holland to Harwich. They also have an overnight sailing. I have taken this ferry many times and it is quite comfortable. From Harwich to Holts will take you approximately 2 1/2 hours to drive.
Theoretically, you could arrive on the overnight ferry, which will put you on the road at about 7am. You can then pick up your new rifle, have a good lunch, and motor down to rejoin the ferry for the return trip that evening, at around 9:30pm, with everything sorted out in one day.
Perhaps not the worst idea… it would mean I do not import via a gun shop, but get the papers done myself…. I’ll let them first figure it out a bit, and if it seems too complicated, I might try that too…