Amazon have the Ladybird or Bat Toddler Safety Backpack with harnesses straps available for £1.79 & £1.99, with free delivery. It's from a third party seller so there's no minimum amount required. You'll have to click on the links below, then go to 'other sellers'.
I used to be dead against putting toddlers on reins, until I had a toddler, and then two! I suddenly and quickly became a fan. I didn't have my girls in a pram, but I did have them close. I have seen some people with complaints about the quality of these backpacks but we've had the ladybird for two years, and haven't had any problems with it.
We have the ladybird backpack and my two year old loves it uses it running around the house (holding her own parent handle!) more than we do when out, and I can tell you three things about it:
1) It's a really small backpack, in comparison to some better known brands. It's a great size for a very small toddler - like a two - three year old - as they can't put much in it. It's perfectly sized for their snacks on a day out though, or a nappy and small bag of wipes if you want to head out without bags.
2) It's perfectly functional, and it works well, but it's not sewn as well as some of the more expensive brands, but in two years of use, we've never had a problem with it.
3) If it's something you're going to use every day, you might want to invest in something more expensive, but if you're going to be using it occasionally, for a holiday, or just to have a spare one, they are superb, and at the price, I think you can't go wrong!