There are a number of good places in the US, but your main problem would be the shipping cost to the UK.
Stan Cook,
mrcook4570, does pen box engraving.
What would be your top 3 or 4 for quality and value in the US?
I personally have not dealt with any for engraving pen boxes, I've been told that Stan does a great job. You can also check with
WoodTurningz,and
Wood-N-Whimsies and see what they could offer.
I personally uses
these from Exoticblanks and use my laser jet printer to print my business name on gold foil labels like
these and place the label on the box as I need them.