How do I pay for something anonymously from the UK using bitcoins? I've seen the following guide but it's no good as I can't use bitinstant from the UK. Can someone recommend an alternative?

Step 1.
Download and install TOR

Step 2. 
While using TOR create a Tormail account 
You can confirm the address at http://tormail.org/

Step 3. 
While using TOR, visit http://www.bitinstant.com

Step 4.
Choose the Cash deposit to Bitcoin to email option.

Step 5.
Follow the instructions on Bitinstant to make your cash deposit, and have your Bitcoins sent to your new Tormail account.
No ID is needed.

Step 6.
Receive your Bitcoins via email.
Buying coins non-anonymously in the UK:
Transferwise to Bitstamp's European account - £1 transfer fee, then &lt;0.5% commission to buy. 

Spending:
I guess you would use a mixer service