You will have to pay import
You will have to pay import duty and VAT unless it is something very cheap (£50ish). Tony is almost right, duty is about half on bike components than it is on complete cycles so it is best having the bike listed as components and not as a complete bike but sending 2 packages might prove more costly.
This company is the cheapest I have found for most things.
http://www.parcel2go.com