The Bestway Swim Safe Baby Support Seat Swimming Aid is on special offer right now at £3.25 instead of £10.95 at Amazon, fulfilled by net_price_direct.
(This link will take you to the main page on Amazon. Click on the sellers to the right for this price!)
I've used one of these for my baby at our local swimming pool a couple of times, and found it really brilliant; she was confident and secure in the water, and I was able to take care of her in the water but also be mobile enough to interact with my older kids too.
Of 82 customer reviews on Amazon's website, 56 rated this product 5 stars. Here's a fairly typical comment:
"Great floatation device. While my 1 year old needed convincing to go into it while on holiday, once in he loved it. Found it so comfy he fell asleep in it several times. Headrest is a great feature. Very stable unit -even with my husband pushing it (and my son) vigorously across the pool. Would recommend!"
Delivery costs £2.50 so you'll pay £5.75 in total but that's still significantly cheaper than anywhere else I can find it online.