I'm an ex-Royal Mail manager. There may well be extra screening on packets to and from the UK for security reasons, but I doubt this would add more than three or four days to the usual 7-10 days transmission times. Also remember that many packages from the US are sent bulk air frieght for economy reasons to a European distributor (could be anywhere, Holland, Germany, Scandinavia) where they are then put in the local post. This can add a week or so to transmission times. I imagine the same goes for packages to the US.
The only excess charge should be customs duty. Always best to make sure your order value falls into the free of duty range. Not sure what this is now, used to be £18.00 although many items are not charged you can never be certain.