OK, it’s not a fine, it’s an invoice for payment which can be pursued under contract law.
You are right to ignore Trace.
Wait for a formal Letter of Claim and come back here when you do, this may lead to a process involving court, threats of “CCJs” which are misleading, and so on. You will get help, advice & guidance here if this happens.
In the meantime, send a Subject Access Request to Alliance for all that they hold on you, which will include the original PCN. Its contents may be important to any defence.