To be fair, ASDA were never heavily supporting the Wii U to begin with. The stock they have kept for the last 6 months has been paltry, and I've never actually seen a Wii U hardware unit in any UK supermarket aside from launch month. This is not to absolve Nintendo of blame for handling what should be a very important market very badly though.
I bought my original Wii in an ASDA store in Liverpool back in 2006 and there were queues. Not massive ones, but midnight queues all the same. They were sold out in local game stores, and it was literally the only way I could get a launch day unit. The shelves were well stocked with hardware from all three manufacturers and the shelves had plenty of accessories and games.
Customers in supermarkets are not your average customer that visits a GAME store. You get parents and children going in to do their weekly food shop, regularly exposed to your brand and advertising. GTA V is heavily advertised in all of the supermarkets I go in, because basically, Rockstar is smart.
UK supermarkets have changed since 2006. Big time. There is very little hardware actually stocked, and a lot of DVDs, Blu-Rays and games. With regards to Nintendo, its a joke, and its been like this for months... even 3DS stock is ridiculously poor. I'm not sure the folks back at NOE HQ in Germany (Shibata and co) actually realise what a shitty situation UK shops are in with regard to stocks of Nintendo product...
Here's Xbox 360... PS3 has similar shelf space:

and here's Wii AND Wii U. I've highlighted the amazing quota allocated to Wii U.

Their presence in the likes of GAME is small, but its probably normal-ish for a new console that doesn't have a lot of games. This on the other hand, is unacceptable. Between this and non-existent advertising, it's amazing people know it exists here at all.