I would make at least a reservation or 2. With hostelworld.com, if you book something you just pay 10% of the total fee (usually ends up being £20-£30 depending on the time of the year, where you're staying, and how long) and pay the rest upon checking-in. So you're not spending all your money up front and making it look like you have none when you get questioned by the IO and you also have the confirmation from the bookings to show that you do intend to travel and spent the money on a deposit.
So really, it's worth the £100 or so in deposits to show one booking for each month you're there. And you can always change them if you decide you want to go somewhere else or do something else. Trust me, 6 months is a loooong time to travel. You'll be happy to have something to look forward to. And you'll be really really happy to prove to your IO that you're not going to be trying to settle in the UK.
edit to add: If you have any other questions about backpacking, feel free to pm me. I did my 2 months last summer, so my experience is very recent.