Liverpool St is in the heart of the financial district and is pretty dead at night, unless you fancy heading into the East End (and it's not like Eastenders).
I believe the train for Luton (short bus transfer) goes from Kings Cross, and Hammersmith and City Line goes to Upton Park. That would put Euston Square/Russell Square area as probably the ideal area for you. It would be close to the transport links, and only about 15-20 minutes walk (or a couple of stops on the Picadilly Line from Leicester Square/Soho etc with lots of entertainment and restaurants, and Oxfors St is close at hand for the shopping.
If you are staying in that area, the Busabi Eathai off Tottenham Court Road is fantastic Thai food.