It is a big mistake to send this Fedex or UPS.
Your happy recipients may be faced with their outrageous "brokerage fees" especially since they live in UK where customs exception is very low I believe.
Your only choice would be to lie on customs declaration form, but then the insurance would be invalid, not to mention you'd be breaking the law.
Just a heads up dealing with mail carriers as part of my business for almost 20 years. I know them all.