I have fond memories of Mercury, around 94/95 I think.
I was on the South Coast, my then girlfriend lived in Bradford. Even off-peak, long distance calls were expensive. I signed up to Mercury, you received a code to dial. The savings were so significant that in one evening, I'd saved more than the monthly/yearly (I forget) subs cost me. I can remember being astonished at this.
Soon after this I had dialup internet at home - late 1995 I think, with Demon Internet. This involved a call to London! BT charged a little less for this than usual long distance, as it was London, which came under a reduced rate (something like "long distance on low cost route"?). But Mercury was still significantly cheaper for this, so I set my Windows 3.11 (later Windows 95 - it was the future) machine to dial the Mercury prefix for me before dialling Demon Internet.
I bought myself a Mercury phone as well, so I had the blue "M" button to make long distance calls more convenient.
I can't now remember : did you have to dial the access code, then enter some kind of id code, and then the number you wished to dial? Or was it just access code (3 or 4 digits?) then the number to call. If the former, then the blue button would make more sense.
I also liked the itemised billing that Mercury sent you, it seemed space age compared to what BT offered at the time.
(This was only a few years after VAT rose to 17.5% and BT billing systems couldn't handle the decimal point, having to issue bills rounded (up?) with a credit paid back later.)
This is all from a consumer's memory, mind, so I may be mistaken.
Ian
[Modified to add:] I'm reasonably sure you dialled an access code, then you had an id / account number that you dialled, possibly followed by # , then the actual telephone number you wanted to dial. So the blue Mercury button was convenient, as you could program it to do the prefix and id number for you, leaving you to only have to remember the number you wished to call.