It's absolutely fine to leave the terminal and return in your case. Just leave at Heathrow, do whatever you need to do, and return in time for your MAN flight.

"Ready to Fly" is effectively a marker that you are checked in and airside, let's say you went through the connections route for your MAN flight. If you subsequently left the terminal, and then turned up for MAN check-in/security, that's when it would cause problems because the marker would have you airside. If you just leave Heathrow without going via connections, all is good, and you won't be marked "Ready to Fly" until to go airside on your return.

"Conformance" is simply the rule that you must present at security no later than 35 minutes before scheduled departure. If you fail to, you may be offloaded. There's some subtleties that don't really matter, it shouldn't be of concern unless you plan to get back for you MAN flight very close to departure.

Luggage will go through to MAN, whether you leave the terminal or not.
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