If you go on the NHS Choices website there is a great tool to allow you to create an account and build your own really detailed birth plan. You can then print out several copies and keep them with you.
At our parent craft classes they pointed out the section at the back of your handheld notes which forms the basis of a birth plan it also gives you lots of questions to ask your midwife so you can make informed choices about different elements.
What should happen is that your 34 week midwife appointment should be a much longer one and she will go through your birth plan with you at that point, but if you start thinking about it well in advance it gives you time to do some research and ask questions so you can make the best choices for you. Also, if you're a high risk pregnancy then this might limit what your midwife/consultant will want you to do - but remember everything is still your choice.
So I would say it's never too early to start thinking about it, but you'll want to have really done your plan by your 34 week midwife appointment.
Hope that helps.