To save you some searching and a masive panic attack here is the easy way to do it:

iconv("UTF-8", "ISO-8859-1", "£");

You could also use the same command to do the whole text rather than just the pound sign itself. Example below:

You can use the iconv function at any point, however probably the easiest solution is to put it inside functions of the class you use to extend FPDF:

class myPdf extends FPDF {
    [...]
    public function showMoney($money){
        $this->cell(10,10,iconv("UTF-8", "ISO-8859-1", "£"). $money,0);
    }
}