Yes Sukey I'm afraid Violet, Bertrand, clam and others are right. And anyone who thinks it's a regional thing probably just doesn't know any truly smart people from that region.
My family is quite smart (titled, but only three generations ago so we're not true UC aristos), brothers went to Eton, parents have a small estate/manor house. DF says wireless, telephone and looking glass but it's generational - our generation would say radio, mirror and phone without blinking. We all say gum boots
but wellies is also fine. But definitely not toilet, serviette, settee, couch, ever so, pinky, lounge, perfume. Turquoise is pronounced turk-wires. Ascot is AScutt (stress on first syllable).
A quarter of my class at boarding school were titled or from titled families. None of them said pardon wherever they were from (from all regions), except one local 'new money' day girl. We all knew her parents were Carole Middleton types because of that, but no-one was mean enough to exclude her on such a petty basis. As PPs have said, anyone who did that wouldn't be truely UC, but a bit jumped up themselves.
The City Bullingdon club types are definitely like this which is why I instinctively dislike the city. There are definitely people who send there kids privately to get 'trained' on this suff. My kids are at state school but can speak the UC dialect and 'pass', but I wouldn't want them to go for that type of job anyway... Anyway as PP have said, even if you learn the dialect, there are a host of other small indicators which would give you away at some point anyway. Eg not wearing a carnation ever in your button hole. How many buttons you have on your jacket sleeve.
DH went to state school and says pardon so for the DC we have compromised on "sorry" as despite not really giving two hoots (or I wouldn't have married him!) it's difficult to undo years of ingrained correction. My best friend also says pardon - I judge people by what they do, not what they say.
Jilly Cooper's book Class is a good read for anyone who's interested in this kind of stuff.