Have you asked or looked for what the Tesco policy is? Its normally contained in a staff handbook, which they will inform you about at training. I would read that first so you understand what they will and wont allow. As you say you have already booked the ticket and you have decided not to return after 4 weeks whether they like it or not. If this was in palce before you started the job, then you should have made them aware and if you did it after you started, then thats a risky thing to do, especually as you havent been there a month yet.
Go and check the handbook.
I cna only see them having to nonour the pre book if they knew about it and agreed it was ok.