Before the release of the iPad in UK you will not be able to use the UK account on the iPad itself. You can use it on your computer to buy apps and music for now. Your purchases can then be synced over to the iPad. Once released in UK this should not be a problem.
iBooks is US only atm but it is said that there will be regional ibook stores upon release. I have not heard anything more than that.
To get your hands on the iBooks right now do the following.
Start iTunes.
Scroll down to the little round flag in the right corner.
Choose US.
Create an US account. (Buy US giftcard on
Ebay first as you will need it to create the account and buy things unless you have a US cc)
Login to that account on yout iPad.
Shop away! at least until your giftcard money runs out
Also apps synced over from the iPad with your UK account can only be updated via itunes on your computer and not on the iPad itself. The US account apps will work as usual.
I hope that helps.
/Zorb