I've benefited so much from HUKD and the generousity of this community, it's my turn to post my first deal!
This Silentnight Luxury Collection memory foam pillow is £9.49 at Ocado https://www.ocado.com/webshop/product/Silentnight-Memory-Foam-Pillow/304468011,
the cheapest I've been able to find anywhere on the internet (I assume because of the relative difficulty of returns to Ocado). Retails at £14.99 on Argos (link below).
Ends up as half the price at £5-ish with a groupon deal stack (see first comment by AbandonedTrolley on the deal at http://www.hotukdeals.com/deals/40-ocado-voucher-incl-3-month-free-delivery-new-ocado-customers-only-groupon-using-2515185)
if you buy this with the rest of your £40 Ocado shop (which you've paid £19 for).
1) Create new Groupon account, find Groupon deal for £40 to spend on Ocado groceries plus three-month ‘free delivery’ smart pass for £29
2) Enter code SAVE10 to take £10 off the deal, giving you £40 of Ocado groceries for £19 (half price)
3) Use the groupon on Ocado to buy as many £9.49 memory foam pillows as you need, along with other groceries (lots of good offers under the "half price" section at Ocado), giving you the price of around £5 per pillow (I'm too lazy to do the maths, but it actually works out to £4 something per pillow)
The reviews for this pillow are bad on Ocado, but the pillow is the exact same one sold for Argos at £14.99 here, which has much better and more extensive reviews: http://www.argos.co.uk/static/Product/partNumber/3078981.htm
(The picture on the Argos website shows a slightly different pillow and hasn't been updated, but it is the EXACT same pillow as the one on Ocado - I've bought that pillow on the Argos listing myself previously and it ended up coming in the blue packaging you see on Ocado, so it is definitely the same pillow). If you don't believe me go buy the pillow at Argos yourself.
WARNING: this pillow is not for everyone - it is firm and while made of "memory foam" doesn't completely mould to the shape of your head. But I like it because I'm a side sleeper and squashy-er pillows give me a chronic neck ache. It's a really big pillow and barely fits into an average sized pillowcase. Being made in the UK is a big plus and the cover is really high quality - but all down to personal preference of course! In my experience Silentnight products can be hit and miss, so if you get a pillow that doesn't fit this "firm" description then you might have a dud and need to get it swapped.