2 options if you insist on the champagne name - you do pay more for the name - go to makro and get their own label Veuve Pelletier - �7 a pop and excellent value - a lot of restaurants seel this as a house champagne at �25 + a go.
Much better option - get a bottle of one of the award winning British sparkling wines. They keep winning awards and beating all comers and are excellent value - just can't be called champagne as this a protected name for wines from that region. Otherwise they are often far superior.
Have a look at a local Majestic - they have selection and can advise, or see English Wine info online at:
http://www.englishwineproducers.com and give it a go . . .