The IPA was set up for those of us who didn't want to be associated with BALPA for whatever reason, as an organisation that could offer services to pilots i.e. advice on employment, legal advice etc. The IPF (a union) was established to exploit the privelages of union membership i.e the automatic right to representation when in dispute etc. Much friendlier, cheaper and less confrontational than 'the other lot'.
Despite being much smaller, they have been very effective in representing us on such issues as flagging out and the proposed EU FTL's. Indeed they prevented the hiring of foreign pilots by BA through representation to the DTI despite the proposed deal being approved by BALPA!