I second that comment from @Brocklehurstj . I popped a couple of cases into reserves when they were £140 a case just around Christmas/New Year time. It’s a quality Champagne and should not be confused with a lot of ‘own label’ stuff.
A couple of exceptions to that btw (as I am a complete Champagne tart).
Sainsburys Blanc de Noir (100% Pinot Noir) which can be bought sometimes at stupid money with various promotions, 25% off deals and coupons. I think my record was £7 a bottle a few years ago. Needs at least 18 months age and is then to die for if you like that sort of thing.
Tesco and Waitrose’s Premier and Grand Cru offerings. Prefer the Vintage ones but the NV is not bad cellared for a bit (it’s that word again!).
Aldi’s cheapster is not bad with some time but I find it pretty astringent without aging. The rest of the £10 bunch, wherever they come from, I would not wash the dog with personally (unless serving as Bucks Fizz to a bunch of teenagers at a Sleepover and those days are over!).
When you can pick up a fantastic Cremant du Jura from Aldi for £8 ish, why would you want to drink horrible cheap Champagne?
As a final observation, I know that a few people don’t rate Champagne or the grapes that it is made from and that’s fine by me. However, when I am sipping my aged (and lovingly stored on the plastic crates) collection over the next 20 years, I beg to differ!