There is never priority boarding on domestics out of T5 - I've done LHR-MAN out of T5 countless times and cannot remember it once.
It's ok though, as once you've wandered down from the lounge, most of the PAX are still seated, and if you can be bothered to stand up for 5-10 minutes until boarding is called, you'll be one of the first on anyway.
Going MAN-LHR though, if you leave the lounge at the first boarding call, you'll find yourself right at the back of an extremely long queue. There's no priority lanes, so you can either shove in, or be the last on. Generally I go for the latter option, although it's never much fun as these flights tend to be heavy on hand luggage.