Unfortunately you have highlighted a major problem with Part P and the failings with its inception. We all know that favours are used in all trades, but when that trade is regulated and people who embrace the regulations and pay extra for insurance and notification are undercut by favours from those who are not registered we find the problems you now face. The answer to your question is no, you can not sign any kind of Electrical Installation Certificate (EIC), what you could do, if you can prove competancy in a court of law, is sign an Electrical Installation Condition Report(EICR). There is a saying "you get what you pay for", and in this instance you have paid good money for nothing worthwhile, any Part P registered electrician would have provided you with a valid certificate that for a domestic installation would have lasted for at least 10 years and you would not now be asking this question but spending the money you have just gained from the successful house sale.