To be honest, if you can't be arsed putting in the effort, then a relationship isn't going to work, and there's no point you (and her) wasting time attempting to carry it on, as it will ultimately result in failure. And from where I'm standing (sitting actually), it seems that you no longer feel as though you want to put the effort in.
However, before you call it quits with your girlfriend, I think you need to have a think. Although, to be honest, when a relationship starts to deteriorate, and one party feels it is no longer exciting/stimulating/worth being in, the solution is usually to inject a little bit of the honeymoon period back into the relationship, and change things round a little. You, however, already seem to have had a crack at this (the London trip); which doesn't seem to have produced results. So perhaps you are ready to move on.
Obviously your girlfriend shouldn't be expected to wait around for you, but I suggest that you take a 'break' from your girlfriend and this other girl. Get yourself sorted and clear your head. In this free time, think about what you really want. Do you find yourself missing your girlfriend and the security of your partnership? Do you just find yourself eager to set the ball rolling with your colleague? Do you relish the time out and decide the single life is best for you?
Without this personal time, I don't believe you will clearly be able to see what it is that you truly want. Obviously it's a risk, but what is the alternative? If your girlfriend values your relationship, and loves you and is prepared to make sacrifices to keep you, she will be accepting of your need for space. You must let her know at the end of it where she stands however. This new woman that you like, well, she's hardly going anywhere is she? And if she likes you, as you say she does, she too will be prepared to wait a little...
Either way, you're not terrible - we can't help our feelings. Maybe you and your girlfriend have outgrown each other. Maybe it can be solved. You just need to time to decide which one it is.