slim n save ones are good cause they're quite low in cals (im not a fan of them, they seem to have a lot of milk powder or some kind of dairy which im slightly intolerant to)
I like Exante ones personally, at 200 cals each, but they're big enough to split into 2 portions. So you could theorectically have 4 "meals" for 400 cals. Tesco's own are ok, quite cheap if you're on a budget, and if you make them without the milk they're a lot lower in cals.
I did shakes for a week, didnt get on that well because I was constantly phelgmy with slim n save shakes and just found shakes to be expensive for calories, when you can make yourself a smallish meal with 200 cals.