I think Canon basically assume that most serious photographers will replace the kit lens quickly and most non-serious photographers wont mind because, bad as it is, it's still better than all but the very best compact lenses. They got pretty heavily slated for having the worst of a bad bunch when it comes to kit lenses, which I think is why they've tried a bit harder with the IS version.
Thats true & without doubt the tamron is a better lens than the IS kit version - but maybe not £150 better.
Canon has 2 good consumer lenses now, the 18-55 IS & the 55-250 IS - which cover a good range for most things an amateur would shoot.
You should be able to pick up both lenses for about £250 (£100 & £150 each - £240 for both on camerabox).
Im a bit pissed off I had to spend my money on car insurance - I was so close to ordering the tokina
(
btw - have you seen the prices on flektagon's these days... £150 on ebay, im tempted to sell mine.