That is only because the price was set when the exchange rate was higher. Apple keep the same price for the life of a product from the day it is released until it's next update.
The UK price of the iPad will be determined by the current exchange rate, not the price of the 64GB iPod touch which was set back in September when the Pound was worth $1.68. On the day it was released, the 64 GB touch actually cost £20 more in the UK.
Okay, fair enough. I hadn't realized the pound had gone even lower since then. (Damn how I wish I'd invested in dollars when it was 2:1)
But I'd still be very surprised if the 16GB WiFi version is more expensive than £399. But I guess we'll find out soon enough.