I did this, shipping a Honda from San Francisco to London.
The shipping agents can remind you of insurance companies - they have a whole range of prices, many seem made up on the spot. You are strongly advised to check what their 'sister' agent takes at the other end ( in England ),as this can be outrageous. I went from around £1000 to £250 all in, for example.
You can deal easily with customs yourself, saving more money. If you have lived in the US or been out of the UK for more than 6 months ( check - may be a year ) the bike will not be charged VAT or any toll. Otherwise they run you VAT and duty ( around 8% ) of the wholesale value in Britain PLUS VAT on your shipping charges!!!!
Then you register via the VRO. Again, you need to get it classed as a personal import to avoid bureaucratic ( expensive ) tests. With me, as the bike was over ten years old all these tests were not applicable. This information should be on the DVLA website, or in brochures they can send.
My advice would be give yourself loads of time to check out many agents. Add up all the figures and check the customs attitude so you don't get nasty surprises.