It looks like the 16GB 3G variant of the iPad is now being offered on T-Mobile for UK customers. Depending on what plan you choose, you can snag a bit of a discount on the actual iPad.
New customers can get 1GB of data + 1GB of quiet time data (data used between midnight and 10am) for £27.00/month which will bring the actual iPad cost to £199.00. Existing customers can get the same data plan for £25.00/month and receive the same £199.00 deal on the actual device.
If you'd prefer to go no contract, there are 3 pay as you go plans you can choose from -