You could say that you are coming here to break up with your American girlfriend because doing it over the phone is just so wrong. Tickets cost so much that you figure while you are here you may as well go see Death Valley or Yellowstone or something. Then the next time you come they can look in your file and confirm that you do not in fact have that American girlfriend anymore.

Basically you already screwed yourself by telling them you had a girlfriend here last time. Just be prepared to tell them how much you hate the US and love the UK. You love your job. You love your house. You are very close with your family and couldn't imagine spending much time away from them. You might want to actually make up a list of all the things that suck about America and Americans. Shouldn't be too difficult. You could start with the lack of travel freedom in the US.

You could tell them that you also have a girlfriend in the UK and have a female friend or sister be prepared to answer the phone and vouch for you if immigration tries to call your bluff and phone the UK.