In reply to demdyke:
Have an amazing time - it's a fantastic adventure. I did it with 4 others in September this year. We took 14 days and largely followed the route in the Cicerone End to End guide (
http://www.cicerone.co.uk/product/detail.cfm/book/670/title/the-end-to-end-...
We went at a fairly relaxed pace and had a couple of cafe stops each day and time to relax in the evenings. We didn't have as much time as we'd expected for things like looking round the Eden project, or castles en route, so if you want to properly sight-see then maybe take a bit longer or accept that you'll have longer days. We stayed in youth hostels and B&Bs, spending around £15-£20/person/night. You do end up spending quite a lot on food due to the amount you need to consume, but you can cut it down by stocking up on cereal bars etc. in supermarkets and cooking your own breakfast and dinner when kitchens are available. Wetherspoons pubs were really useful for cheap breakfasts, and we found plenty of nice pubs doing a hearty dinner for not too much.
We used saddle bags/panniers and kept the weigh down by replenishing water in cafe stops and taking one spare set of cycling clothes and one set of evening/sleeping clothes. The heaviest thing was probably bike locks. Most places with bike storage (recommended in the Cicerone guide) also have laundry facilities. We also had some friends meet us in the middle weekend, which was a good opportunity to offload unnecessary things and to get fresh clothes and some luxuries.
Being in a group was helpful for cutting accommodation costs (eg. private room for 4 or 6 in a youth hostel), laundry, and taking turns cooking breakfast/dinner. It was also good to be able to split into smaller groups if people wanted to go at different speeds or some wanted to chat and others didn't. Beware though that the train down to Penzance only holds 6 bikes and they need to be pre-booked. Booking opens 3 months in advance and it's well worth doing it that early for peace of mind. We saw people being turned away on the day.
Cornwall was by far the hardest bit, so don't be disheartened! If you start there and can get through the first two days, you can definitely do the rest.
Have a great time!