The couriers like FedEx, UPS, ParcelForce etc will assess duty and VAT themselves, almost always assuming something has to be paid. Some get you to pay before releasing the parcel, some send a bill later.
Items that come via standard Royal Mail via your postie go via Mount Pleasant first to assess and some stuff can go through without charge when in theory it should have had one. Only stuff I’ve had from US that goes this way is if they send it via standard USPS.
If fees due, they all slap on a handling fee to take your money.
If you want cheap forks etc, try France. Alltricks for example. Or Germany, like bike-discount.de