Unless the underneath of the vehicle suffered damage, the sump plug would not fall out of the sump unless it was incorrectly torqued up during the service or the plug itself was damaged on installation, and even then this would be an unlikely scenario. It is likely that the dealer is refusing the warranty claim on the basis that the chances of the issue being due to a component failure are very slim, if not impossible. If the dealer is unwilling to change their minds then we would suggest making a complaint to Honda UK, explaining the situation and the response you have received from the dealership. A closer examination of the vehicle may provide clues to the cause - it is possible that the sump plug was cross-threaded when installed, but unfortunately it may well be a case of his word against theirs.