Liverpool Airport is quite busy these days, with based easyJet and Ryanair aircraft, plus a number of charter and cargo airlines passing through each day.
A recent visit allowed me time to check the place out for spotting locations. I found the following:
Inside the Terminal. Upstairs in the arrivals section, there is a food court. This has windows overlooking most of the main apron where airliners park.
Outside the Terminal. Turn right into the car park. From here you can see a couple of gates not visible from inside.
GA Car Park. Walk or drive along perimeter road to the right after leaving the terminal. You will come to the General Aviation car park, from where you can log aircraft parked there.
Speke Business Park. At the old airport site to the north west of the current terminal, there is now a business park. The old terminal is now a Marriott hotel. Outside it there are two aircraft:
G-JMAC Jetstream 41
G-ANCF Bristol Britannia