Please remember bit coins are not illegal and can now be used to buy all sorts of things however I'm not sure how BTC are purchased in the UK to be honest so can't offer any informed opinions. When it comes to placing an order NEVER send any identifiable information that has not been encrypted and yes getting a bootable USB running tails is a good option. Anything on your computer itself should be kept in an encrypted vault e.g. trucrypt making sure you set up both the inner and outer volumes so if 'pressed' you can give up the password to the outer volume where the non incriminating information is saved.