Prosecco and cava are completely different from each other though. Not saying you can't get nice of both, you can. It just can't be called Champagne due to where it is made.
Prosecco is much sweeter, your cava tends not to be. You can get rough lots of it all, you just have to taste your way through as much as possible and find what you like!
I think the new Tesco double clubcard points offer this time around included their 'Finest' wines and Champagnes, so you could get either double the quantity or up the quality if you are a regular shopper with them.
Tried this one last night. It was on offer in the shop and I thought it was worth a shot for a tuesday. Wasn't too bad, but a bit sharp finish. Worked out about 20euros on offer, which is cheap champagne, but I think I'd still take a Mercier over it though.