I'm pretty sure the standard duty on imported bike parts is 4%, though it's 14% for complete bikes. This is based on past experience from importing into the UK (When part of the EU) from countries outside the EU.
The import price that the duty is applied to includes the shipping. The VAT just applies to the goods. (I think I have that right)
The handling charge will be a set fee rather than a percentage, I've paid anywhere between £9 and £25.
I think that's your worst case scenario.
There is a lower rate, I think 2.5%, for some goods you are carrying yourself, someone told me this also applied to items posted, but I'm not sure they were right.
Importing into the UK there was also always a chance it would just get missed, despite having the correct customs declaration form clearly visible, you have to just treat such things as a bonus when they happen.
EDIT - Cross posted with Brush2805, I might go try that, see if I was right!