I needed a self inflating mat for my daughters summer camp. I saw this one on Tesco Direct a few weeks ago when it was £14 but OOS. Checked again today and it's back in stock and reduced to £9.80. Compared to the other sites I looked at this is the cheapest.
Free delivery via Click & Collect otherwise I think it's £3 which still makes it cheaper than all the other sites I looked at.
I'll let you know how comfy it is!