Bump. I would be interested to see some replies on this from people with experience in the field.
I've been looking at GPRS too, primarily so I can use ssh and do server maintenance while out on the road in dire emergencies.
One thing to watch out for will be costs. It sounds like your bandwidth usage will be high, so go for a plan with lots of included bandwidth. Some PAYG networks charge a flat 7p (or so) per kilobyte. If you get 40kbps, say, you could be paying 35p per SECOND, or £21 per minute ;-) Very scary stuff.
The coverage of 3G is too poor for you to consider yet, especially if you're going to be on the canals.. but you can check the latest on three.co.uk.
If going the GPRS route, you will either want to use Infrared or Bluetooth. Infrared is the cheapest option as most notebooks already have this.. even my 4 year old notebook can work this way. Bluetooth is supported by many new notebooks, but can be added with an addon card to most older ones. Then you set up a GPRS modem connection on your PC, get your phone within range, and off you go.