That one's not self-inflating. You have to blow it up, but it's different to a normal airbed as the air compartment is divided into hundreds of little linked pockets, which means it will keep you off the floor with less pressure/volume. I much prefer this type for festivals as they pack down to a truly miniscule size so you could take a few. Self-inflating mats, like the original Thermarest, don't pack down anywhere near as small.
Personally, for Glastonbury, I'd wait until Lidl sell their own 'Crivit' branded version, which will probably be quite soon as they usually do a camping sale in spring. These are orange things that look a bit like a lilo, but don't cost much more than a tenner and do the job really well.