I was thinking about going to the Milton Keynes snowdome myself, and I had a look at their website.
They currently have an offer on with Tesco, where for a package of four 1 hour group lessons you can use your Tesco vouchers which are worth four times their face value. So that, as far as I can see, for the adult group lessons which currently cost £100 for 4 hours you can get them for only £25 worth of Tesco vouchers. But you do have to order the lessons ahead of time and sort out the vouchers, you can't just turn up with your Tesco vouchers and expect to get the lessons cheap.
This also applies to their other snow domes at Braehead and Castleford.
http://www.tesco.com/clubcard/deals/product.aspx?R=899
And, the other thing that occurred to me, was that beginners might not realize you need different clothing for snow domes and outdoor mat dry slopes. Snow domes are cold - the one at Milton Keynes is minus 4 degrees, and you need to wear ski gear, like salopettes and ski jacket, woolly hat, ski gloves and ski socks (although you can hire ski gear at Milton Keynes).
Whereas, if you are ski-ing outside on a mat you are subject to the current weather conditions - like rain or sun - and you need to adapt your clothing to this. However, even in mid-summer you still need to wear clothing that will protect you from getting scraped on the mat, like gloves, long sleeves and long trousers.
Ally