No one has mentioned temperature yet, and it is one of the most important things to get the carbonation correct....well, UK real ale correct.
In a non technical nutshell, a cellarman stuffs a new cask in the cellar, lets it settle and get down to a temperature of 13 degrees. He then does the tap and bungs stuff to slowly let the excess pressure out and give the beer time to de-gas down to a fizz level that is known as 1 volume per volume. This carbonation level happens naturally at 13 degrees as long as the gas that is above the beer is Co2 and nothing else. That is why pub cellars are 13 degrees. 1 volume per volume means that a pint of beer has got 1 pint of Co2 dissolved into it. (If you had the cellar colder it would taste fizzier, and if it was warmer it would be flatter.....)
This is weird, but theoretically, if you then poured a pint, stuck cling film over it to trap the co2, and left it in the cellar, at this magical 13 degrees, the carbonation level in that pint would stay fine and dandy forever!!!! (as long as no other gasses got in through the cling film)
I say theoretically, because someone always comes along and drinks it before the results are in......
So, at 13 degrees, and with pure co2 in the airspace, your corny needs only atmospheric pressure to stay "real ale" fizzy, There must have been a point where the beer in the corny was carbonated to a level higher than 1VpV and that should have been achieved during secondary fermentation/ carbonation. You will have then done exactly as the cellarman did, and let the beer slowly de-gass down once it is in the 13 degree state.
Now, where do I find the links to using low pressure butane regulators to simulate cask breathers, cos my beer is nearly ready to go in the cornys, and even though I have been practicing for 42 years, I can't drink my beer as fast as CAMRA says I should, so they can far cough!
PS What does CAMRA do to compensate for pubs who are up mountains? Their atmospheric is different by a big margin compared to your seaside establishments, so even their strict rules are a load of pants