The best deal is to get an AMEX Platinum Charge Card (300£), you get, 2 full unlimited priority memberships, travel insurance for you and family, flight delay compensation and a few more niceties.
Reagarding access to lounge you will need a valid boarding card on your name, no matter if it says stand-by on it.
Some lounges will ask you not to enter wearing your uniform or to change.
I have used the lounges about 40 times this year and think is great value.