The W3570 is a redundant model, the W3565 is 3.2GHz also and can be found for $600. You can find the W3580 for around $1,100. Go to google shopping to price search or any big site, Amazon, Newegg, Buy.com
Here in Germany the W3565 will cost you around 549,- Euro while you have to pay 969,- Euro for the W3580. I don't think the W3580 is worth the premium price you have to pay for it.
1. Most stores in the UK don't stock the high end Xeons because the demand is tiny, so you will need to contact a company and they will order it. I'd find one with a phone sales department and try calling.
2. The 6 core 3.33GHz model that is out in a month replaces it so they aren't in supply. Which isn't good news for Mac Pro owners looking to upgrade.
A 3rd party is selling the W3580 on Amazon for £980, but I'd contact LambdaTek first, or even Scan.co.uk, who while they don't list them may still be able to get you one. Your other option is to go with a Core i7 version, but I'd ring round a few places first.