In my experience, International to Domestic transfers through T5 are quick and straightforward. There is rarely a queue at passport control and no customs check - the bags go straight through to your destination untouched by passenger hands. The only potential holdup is at security, but that probably feels longer than it is most of the time. Domestic to International is even easier - one scan of your boarding pass and no security wait.
On my last visit, a couple of weeks ago, I had 65 minutes scheduled between arriving from Paris and leaving for Manchester. I made the MAN flight easily, even having time for a swift one in the EC lounge!