I bought a pack of champagne yeast, but not sure why, just to try it :) What differences should I expect compared to a "normal" English ale yest?
A champagne yeast will most likely be different from an ale yeast in two ways:
While an ale yeast may fermement down to 1.008 or so, a champagne yeast will ferment to something much much dryer, closet to 1.000
Wine and champage yeasts do not typically produce the amount of krausen that an ale yeast will.
I have used champagne yeast to make an ale but never alone, I have added it to very strong high ABV brews to dry them out at the end of the fermentation. If you want an ale flavour then use a standard ale yeast for the first week or 2 then add in the champagne yeast.
Most of the yeast based flavours are made in the first 3-4 days of your brew, so the main flavour profile you want choose that yeast for the first days. But, be warned that some yeasts may clear up the flavour compounds created by others.
Also, where an ale yeast may slow down at 8-9% and die off at 10-11% the champagne will keep fermenting to 14-17% so if you want a potent and dry barley wine than it may be for you.