Yes, shower use at SATS T3 is free (for PP and paid-access alike). I imagine the policy's the same for SATS T2, though couldn't say as I've not been to that lounge in ages. It's a pity what's happened over the years to PP T1, from shower upsells to downgrading of moderately quaffable wine to paint thinner and removing the recliners (in fact, the entire rest area that used to be adjacent to the showers). I'd mentioned it solely as OP inquired about a "quiet lounge" from 07:30-16:30 and SATS T3 can be anything but during that window.