Yes, the 39 weeks is UK law, so you are entitled to receiving it all (6 weeks at 90% of your salary and 33 weeks at about £125/week). You should also be entitled to your company's paid annual leave on top of the 39 weeks, but this will depend on what your company offers.
Mine only offered 20 days (1 month) annual leave, which I took at the beginning, though I like the idea to take it at the end like some of you did so as to relieve your bank accounts after a long period of inactivity! Nevertheless, we were afraid of our initial costs at the start of having our LO. This is the first week of my official maternity leave for one year, so in total I will be at home for a year and a month, yeah!
Though I work for an international (US) company in the UK, I was encouraged by my very kind HR rep to take the maximum amount I could take and did so. She also clearly explained that I could come back to work for a day or two and get paid for each day, even if I only came in for a few hours. My UK boss is supportive, though my US boss was in shock (though couldn't say anything b/c she understands that it's my legal right).
After 9 years with the company, I think it's time to shift my priorities to my LO and enjoy watching his development instead of that of our grantees! :)
OompaLumpa , I'm so sorry that your boss is being an a** about this, and he has no right to treat you this way. If appropriate, you should talk to your HR rep so that they may educate him! 
My only caution about taking a year off is that the last weeks at work become VERY stressful as you try to hand over everything to colleagues! I guess this depends on the individual...
Good luck with your decision, and hope that everyone will be supportive! At the very least, your baby will be! :)