40kg will be pretty expensive. Best bet would be to send it surface mail, although that'll take up to a couple of months to arrive. For this weight airmail is pretty much out of the question.
Also, with 40kg of stuff it's likely to be stopped by customs. This could result in a pretty big customs bill for duty on items.
Edit - US Postal Service has a max limit of 70lb, so 31kg or so. For 66lb you're looking at US$120ish for surface mail according to their website. This is excluding any potential UK customs charges.