In London, you can buy a plastic neti pot at Whole Foods (High Street Kensington) for £3. I was told that they have trouble keeping them in stock because they are so popular, but happily they have just received a shipment of 40, not yet on the shelf when I went in this week. If you do go, apparently they normally keep them in the aisle where the yoga equipment is (huh?), but if you don't see any, just ask because they might still be in the stockroom.
As for the solution sachets, Whole Foods doesn't sell them. However, I saw them at nearly every Boots I checked, and in some cases they came with a plastic bottle (an alternative method if you don't like or can't find the neti pot). The brand is NeilMed Sinus Rinse, and the sachets run about £17 for 120. I did not find neti pots at any Boots or any of the independent pharmacies I tried, even "alternative therapy" ones. Whether they don't stock them, or were just sold out, I don't know.
Hope that helps!