In addition to losing the mortgage interest offset against tax, you'll probably get a less favourable interest rate - on an investment mortgage, you can expect to pay a premium of anywhere between 0.25-0.75 percent over home-loan rates (depending on how good either rate is). In addition, if, as seems to be suggested, the mortgage secured against the investment property would then be very high, you may not be able to get it in the first place, or not without very onerous conditions.
About the only pro of the proposal is that you may be able to dispense with mortgage related life assurance on your current PPR mortgage, but you may not wish to anyway, depending on your circumstances - and any life assurance savings are most unlikely to outweigh the various tax and financing costs.