Clicking the paypal "cancel this and return to previous page" thing clears out your cart -- I figured that out the first time, but luckily there were still seed accounts available when it happened, and now I'm turtling with it until the issue's resolved.

If you're able to (since you're in the UK), you can try picking up one of the second batch of 200, and just leave that in your cart (screenshot it for proof maybe?) until the problem's fixed.