I thought I would jot down a few notes about things I would suggest anyone thinking of doing this should consider. This is from the point of view of an unsupported trip.
Getting to your start point - the easiest way is to travel by train - but book as far ahead as you can (12 weeks is the current maximum.) That way you can get an Advance ticket which depending on where you're travelling from can be a LOT cheaper than the standard fare. Since you have to travel on the same train as your bikes and you need to make a bike reservation at the same time this is the obvious thing to do.
One decision to make is whether to get to somewhere near Land's End (or John O'Groats) the night before or do the extra miles in the morning. If you choose the latter then allow for the extra miles in your first day's plan. Personally I would recommend the former - there are quite a few B&B's near John O'Groats and also a fair bit of accommodation near Land's End (although booking ahead is important.)
Getting home from your destination - This can be a bit tricky! If you are certain you can stick to a schedule then you can book a train and reserve bike spaces - but obviously there is rather more uncertainty. The big problem is the limited bike spaces available on trains - and in the case of Thurso and Wick, there is a very limited number of trains available - 4 per weekday/Saturday and 1 on Sunday. I made the mistake of trying to organise our return travel on a Sunday evening and ended up talking to somebody useless in a call centre. All the train companies seem to use the same call centre. I then tried booking online through the the East Coast Railways website and despite thinking I had a bike reservation, I didn't.
There are a couple of other options - it is possible to post your bike home. The bike shop in Thurso will sell you a box (15 pounds - a bit cheeky since other places will pretty much pay you to take them away...but supply and demand and all that) and help you dismantle and pack your bike. You then take it to the post office about a quarter mile away and simply mail it. It cost me less than 20 pounds including insurance. The bike beat me home! You can also try dismantling your bike and putting it in bags/boxes and putting them on the train. Sandy did this and it worked perfectly for him. Obviously it depends how small your bike will go. With these options it matters a lot less if you miss a train connection (assuming you have an Anytime rather than an Advance ticket.)
It is also possible to catch the Orkney bus to Inverness from either John O'Groats or Wick. The leaflet says they will carry bikes for free - but I suspect it is likely down to how much space they have. Some websites say they don't take bikes - but the photographer at John O'Groats recommended this option to us. That would save pedalling back to Wick or Thurso (about 20 miles) to catch a train. It seems to go twice a day.
Another option would be to cadge a lift from a supported group - we didn't try this one either - but it's quite possible you could get a lift back a good chunk of the way if your arrival timed nicely with a group with a minibus and big enough bike trailer.
Accommodation - Some people choose to camp - this is obviously the cheapest and most flexible option - but it can be quite a lot of extra weight to carry. We met quite a few people taking this option, but personally it's not for me.
Bed and Breakfast - the main question is how far ahead to book. We booked the first two nights' accommodation - the one before we set off in Sennen and the first night of the trip proper in Veryan. Despite booking three months ahead this was much harder than anticipated. The B&B's I tried in Sennen were all booked and we ended up staying in The Old Success Inn in Sennen Cove. This was fine - except it was at the bottom of a very steep hill. The first night proper I had been aiming for Tregony, but again they were all fully booked 12 weeks ahead, so we switched to Veryan - which as it turned out was a good choice.
For the most part we didn't book ahead - although if staying somewhere busy like Kendal I would recommend doing so. I would also recommend it for the quieter or more remote places. We phoned ahead to Langholm and got into the 3rd place out of the 4 showing on the web. The stretch between Lairg and Bettyhill (or Tongue) is also very sparsely populated and other than Crask and Altnaharra there is absolutely nothing in terms of accommodation. The Altnaharra hotel is also expensive so beware.
Money - Lots of B&B's don't take credit cards, so carrying some cash is quite important. Also, lots of places in the north of Scotland don't take plastic either.
Daily Distances - Obviously your mileage will vary - we planned on doing 55 to 80 miles a day with the exception of the last day. We nearly always ended up doing a few more than planned - in one case almost 20 more - but that did allow us a shorter day later on. Most of the deviation was down to the difference in joining dots on the GPS versus actual wiggles and ups and downs. Some deviation was down to varying the route to avoid traffic or similar. Our shortest day bar the last one was 62 miles - the longest 88 (or for Sandy, 92). We had plans to do a bit more sightseeing along the way, but after the Eden project we didn't do a whole lot. Our focus became getting a decent number of miles done early on and not finishing the day too late, particularly if we didn't have accommodation booked. If I did it again I would a) take a different route (the one we did was fine - just a bit of variation) and b) aim to do fewer miles each day and do more of the sight-seeing thing.
What to carry - We started off with three sets of cycling gear, some tools, lights, waterproofs and some gear to wear in the evenings. This didn't seem like a whole lot, but it's amazing how the weight and bulk builds up. We also had a book each and I brought along a decent camera. Sandy used the camera on his phone. We reduced the load slightly in Edinburgh/Dunfermline, but not a whole lot. If you can wash stuff along the way and get it dry in time, then that helps keep the volume of stuff down. We didn't carry any maps - relying on the GPS the whole way. That worked ok, although the maps on the GPS itself were sometimes a problem - although not a serious one.