The employer simply seems to meeting her statutory duty. You were not so much a 'part-time' worker as a 'casual' worker. That means that the employer should 'put you on the books' every time you start a new term of employment in the shop and 'take you off the books' whenever you finish working there. (That includes providing you with a P45, so that you can hand it to another employer, such as if you did some part-time bar work while at Uni).
Sending someone a P45 isn't necessarily a 'nasty' thing to do. It's just required by law. I work for a firm which carries out traffic surveys, with staff employed on a casual basis. I choose to work as self-employed but my colleagues who go 'on the books' often receive several P45s in one month from the employer (because they're 'on the books' for a few days and then they come off again) but they still know that they'll be taken back on when there's more work available.
Contact the employer and ask whether sending you a P45 was just a formality or whether your services are no longer required. You might find that they're begging you to come back ;-)
Chris