This is pretty standard in the UK. Your new full-time job is commonly effectively at-will for the first 3 months, and possibly more.
See, e.g., http://www.jobsite.co.uk/worklife/probation-periods-19677/
See, e.g., http://www.jobsite.co.uk/worklife/probation-periods-19677/
Employment with [company] is "at-will." This means that you may terminate your employment at any time, with no prior notice given to [company]. It also means that the company may terminate your employment at any time, with or without notice or cause. While the company generally adheres to progressive discipline, it is not bound or obligated to do so.
As an at-will employee, you are not guaranteed, in any manner, that you will be employed for any set period of time. No one in the company, except the President, in a written, signed contract, may make any representation or promise to you that you are other than an at-will employee. Any employee, manager or supervisor who makes such a representation or promise to you is not authorized to do so.
For the last few years this has been written into law and employees don't get their full employment rights (such as the right to contest an unfair dismissal) until you've been employed for 12 months. (A de-facto one year probation period.)
I should note that employee rights during this probation period are probably stronger than in many US states.