Ben,
I ship stuff around the world ( both imports and exports ) quite often as part of my work. Mostly it is back and forth between UK and Japan, or Europe and Japan, but I do occasionally ship to and from USA.
For a UK to USA shipment of something that size, I'd recommend PANALPINA:
PANALPINA WORLD TRANSPORT LTD
PANALPINA HOUSE
GREAT SOUTH WEST ROAD
FELTHAM
MIDDLESEX TW14 8NU
tel: 020 8587 9000
You'll have to remember that the actual weight ( you mention approx 40kg ) is not the only factor. You have to take into account the 'Volumetric Weight' too. To get an accurate quote, you will need the item in its crate so that you can get the dimensions........ They won't really want to talk to you until you have the thing boxed up and ready to go. If you have not packed it yet you will have to try to estimate the finished size and weight.
I think something of this size and weight will not cost all that much more to ship via Air than it would by Surface. Its only when you have very high weights or high volume that Surface ( Sea ) freight becomes efficient.
I think you will also find that pre-paying the freight cost on the UK side could well cost something approaching double the cost of a 'collect' payment ( consignee to pay ) in the USA. I find this with all my shipping charges; I even find that pre-paying freight charges from Japan to UK is cheaper than paying them here in the UK. The UK is one of the most expensive countries in the world, and the exchange rate from £STG to other currencies will really hurt your consignee....... If possible, try to get the freight charges charged outside the UK.
Good luck.