If you know the size & weight of the parcel your parts would be in,
you could look on one of the UK courier price comparison websites like -Parcelmonkey.co.uk {there are lots of others}
If you find a good deal on there , then see if the sender of your forks will use the courier mentioned.
Royal Mail{ and Parcelforce} is often the most expensive , but is the most convenient for sender - you just go in your local Post Office.
hope this helps