Controllers and FISOs are encouraged to file a report so that the scale of the infringement problem can be determined. NATS just loves statistics.
Unless you are a repeat offender or the incident caused major mayhem / safety concerns you are very unlikely to be penalised. The fact that you contacted the ATC unit on landing also goes in your favour as it shows that you acknowledge that it happened and that you wanted to explain and hopefully learn from the experience.
Tick it off in your book of "lifes experiences" and enjoy your flying.
