I reserved the cartridges for in-store collection today and surprisingly I also received the Brita Water Jug for free when I collected; its normal price is £20.
The cartridges are item number 840/7045 and the jug is no longer listed on their site but the item number is 842/2910.
No guarantees that others will also get the jug but it was listed as deal for the pair back in April for £20 so its obviously still on their system. If they have stock in your local store I suspect you will get it. Worth a punt at this price.
Previous deal - http://www.hotukdeals.com/item/385528/free-brita-marella-water-filter-jug/