I don't know the exact equivalents in the UK, but if the Dollar Tree (where arguably nothing's just a dollar any more) equates to Poundland in the UK, I would note that it's unlikely that the latter will ever have a self checkout option.
The positive for 'self Checkout is that it takes advantage of a single queue feeding multiple checkout stations. The only other time I can recall seeing such was at a post bank when I was stationed in Germany. It is the fastest way to check out.