Hi Matt,
With limited time and a desire to see some good bits, I'd go for an off-the-shelf route and presuming London would be the start point, then Wales offers a short end-to-end that could take in Stonehenge as an easy add-on. On and off-road routes can be found on this site
http://www.sustrans.org.uk/ncn/map/rout ... ymru-north and of course
http://www.sustrans.org.uk/ncn/map/rout ... ymru-southThere are direct train services London-Holyhead where the ride starts, doing it this way round means you don't finish in Holyhead which is a ferry terminal and not a lot else. Bikes go free on some trains, the service to Holyhead is run by Virgin, last time I used them there was a small charge, others may know better?
To get to Stonehenge just keep following the cycle routes from Cardiff over the Severn Bridge and head towards the Salisbury area. The Bristol-Bath cycle path is on a disused railway and excellent for all types of bike. Plan an overnight in Bath, it's a lovely Georgian city with many Roman remains to see. Prepare to be slightly underwhelmed by Stonehenge, it's quite commercial in the same way as Niagara Falls, I know you'll still go, I went to the Falls, but 'they' were right to advise me not to bother. Salisbury is also very nice in parts and has a direct train service back to London. See
http://www.thetrainline.com/ for times and costs, buying a ticket early and collecting it the station [you use the card you bought the ticket with] will save a huge amount.
Hope this helps, might be good to post your plans when they're clearer, may tempt some to join you on route.
Cheers, Richard