On the most "deal-est" of days we have for you the Seagate STBV5000200 5TB External HDD. 5TB, as in five terabytes, as in 5,000 gigabytes! A gargantuan amount of storage for anyone, 5TB roughly translates to 1,250,000 songs, 1,667 high definition films, or over 2,000,000 photos.
This external HDD connects via USB 3.0 which transfers at around 640 megabytes per second (MBps) which should allow you to quickly grab and transfer or read your files. This external HDD is not a "portable HDD" and requires an external power source to run which may be a turn-off for you. That being said the HDD is small enough to pick up and put in a bag so although no technically "portable" it's pretty easy to move around.
My biggest concern with the drive is the brand. Seagate are notorious for its drives unreliability, and have some of the lowest reliability rates in the storage game (If you'd like to know more check out Backblaze's breakdown of HDD manufacturers here). However the STBV5000200 has 4.4 out of 5 average on Amazon with over 2,000 customer reviews and a rating of "Excellent" from Cnet.
It's a tough one to recommend but 5TB for £120 is a stonking-ly good deal and the device comes with a one year limited warranty. Get the deal here.