A 2 month trial allows you to sample xbox live for 2 months, just like if you had paid £40 for, but you don't get a headset.
What I did was get a 12 month subscription which includes the headset, then use the trial and then the full 12 month code after the 2 months, or you can cancel it.
You can take your xbox live account straight over to xbox 360 live when it comes, I don't see any reason not to be able to take a 2 month trial over, whether that will work if you start the trial on 360, I don't know.