In looking at the other websites, the Panasonic UK Shop is the cheapest for Double sided 120 min (4.7x2/9.2) £24.99.
As for DVD-R, I have tried Jungle's Monkey Media, pack of ten worked out at £6/disk.
Keep an eye out for DVD-RAM on ebay. I took delivery of my Panasonic E20 yesterday and luckily got a great deal today on ebay for brand new doublesided Panasonic DVD-RAM for only £12 each. Cheapest I'd seen them was £24.99 on Panasonic's eshop.
DVD-R I picked up from www.bigpockets.co.uk,the Princo branded one's and touchwood they have been fine £19.99 for 20 plus postage.