WeeJohnyB got it right. New Era run either 'open' practice slots - 10 minute slot open to any solo class; about four or five of these, or 'closed' practice slots for specific classes (they don't mix sidecars with solos - not sure they're even running three wheelers these days). My guess is we will be in 'open' slots with everyone else - so you will be out there with everything from GP125's to 190bhp Probikes. My preference is to let a couple of the sessions go first to help clean the track so I usually aim for the third of fourth slot, then go for it.
You're right about the warm up lap, but then everyone wants to get as much heat into their tyres - your average club race is only 6 - 8 laps (depending on circuit length) and for most people the first three corners prety much decide approximately where you will finish - loose touch with the leading group in those first corners and you will never catch up, stay with them and you can get a tow.
Cadwel woodland circuit is a bit different, 'cos there are no straights to speak off - it's all corners and a bit intense!