The usual caveat is that you cooperate with the company in a positive manner - i.e. you show you are keen to work with them to settle the issue, and don't string them along. (Not that I am suggesting the OP is).Okay, good.
They are extremely unlikely to prosecute you, unless you have been caught many times/are being rude or extremely suspicious to the revenue protection team.
To get the best outcome, the OP should return here when they get the letter, and we can provide help and advice before the OP responds.