If it was genuinely intended that the function would raise money for charity but it failed to do solely because (say) an insufficient number of people attended, then there was no 'false representation', which would be necessary to bring a charge under Section 2 of the Fraud Act 2006.
Of course, if the 'costs' simply went into the oganisers' pockets, rather than paying for things like caterers or entertainers, then an offence would have been committed:
http://www.legislation.gov.uk/ukpga/2006/35/section/2
Chris