As Soof said, the bike shuttle isn't for everyone but i think it's reasonably pricey for those that don't fancy riding down there. I decided to book based on a few factors:
1. I'm taking the missus and she won't appreciate 2k motorway miles on the back of the RR!
2. I factored in fuel costs, tyre wear, added mileage/depreciation on the bike, additional hotels, more days off work etc and decided the shuttle is actually pretty good value.
3. Flights from Luton are really cheap ?55 pp return
The actual cost for the shuttle is ?430 but soon adds up once they add hotels (2 nights @ ?90 per night), bus shuttle service at both ends etc. You could definitely save money if you choose a cheaper nearby hotel.
On a side note, my ferry +cabin to Santander last year cost me over ?500 (!) so i think Bikeshuffle have got their pricing just about right.