On the subject of Manchester security, I was made to feel like a naughty schoolboy at the weekend for being in the departures lounge without my biometrics being taken. Flying LAX to IOM via LHR and MAN. My biometrics had been captured at LHR an hour earlier but the security guard at the entrance to the gate 14x corridor in T3 made me stand in the corner whilst he dealt with the rest of the queue and made out that I had done something wrong and somehow evaded security.
My crime? Getting off the flight from LHR and walking into the departures lounge by following the signage for T3 departures.
I had already had biometrics taken at LHR T5 transfer desk so thought nothing of it until my Flybe boarding pass refused to let me through the automated gate.
If there is a biometric loophole at T3 then fix it, but dont take it out on your passengers by embarrassing them at the gate
Part of the OP's problem is is that the photograph (and it's probably just a photograph) taken in Heathrow T5 is used only in T5 to ensure that the person leaving on a domestic flight is legitimately in the UK. The idea is to avoid the situation where a transit passenger not allowed into the UK swaps boarding passes with a friend who is allowed in and does not mind a brief trip overseas. Heathrow Airport Ltd and possibly the airlines prefer this system to providing separate domestic and international departure lounges.
The same scheme has been introduced in Manchester T3 in the past few months, presumably because some international transit passengers can avoid contact with UK Immigration.
What i don't understand is how the OP got into T3 Departures without being photographed again. I don't recall seeing any signs anywhere on the T3 domestic arrivals path indicating a route into T3 departures anywhere before baggage claim.