Yes you can but if you need to fill a customs form out please mark the package as:
'Controlled Goods - for Temporary Import - FOR REPAIR AND RETURN.'
By marking it as for 'Temporary Import' there should not be any customs charges raised.
If any customs charges are raised when the device reaches the UK or when it's returned and reaches your country mendmyi are not, and will not be liable for them - the charges are the customers responsibility.
'Controlled Goods - for Temporary Import - FOR REPAIR AND RETURN.'
By marking it as for 'Temporary Import' there should not be any customs charges raised.
If any customs charges are raised when the device reaches the UK or when it's returned and reaches your country mendmyi are not, and will not be liable for them - the charges are the customers responsibility.