Why would you buy a phone? It's only the SIM that you want. Most UK operators (including Vodafone) will send you two free SIMs. Obviously they will need to know your address. If that is too "registered" a condition, you might indeed have to get hold of a phone. However, it doesn't have to be an expensive 3G phone. It doesn't even have to work, so long as the SIM is still valid.

An even better deal can be had on eBay, where you can buy a SIM with more prepaid credit than the price you pay for it. It sounds fishy but is perfectly legit. Phones are shipped to the shops in a package including a SIM with pre-installed credit. When an existing customer buys a new phone, he or she usually transfers the old SIM into the new phone and the shop is left with the new SIM. These spare SIMs are sold in bulk to dealers who then sell them on eBay. Market price is usually less than the amount of pre-installed credit, sometimes a lot less.

Trouble is, you have to be registered on eBay and probably on PayPal as well. Is that too hard?

You may be an internet addict, but how much squinting at a tiny screen do you really want to do in a day? Remember that you can get most of your N900 usage via WLAN at home, work, cafe, on the train or sometimes even on the bus, and so you might not use your mobile phone account for much data transfer at all. Without buying any special data package, you can get up to 25MB of data in a day from Vodafone UK for 50 pence or less, deducted from your PAYG balance. Some other operators have similar pricing.