Signed a job offer letter but having second thoughts

Hi all. I was recently offered a new job (due to start a week today), but having learned more about it, have decided that it's not right for me. I haven't signed a contract, but have signed a job offer letter - but it doesn't state whether it's conditional or unconditional. From what I've read, it has to be unconditional to be legally binding. I have a three-month probation period at the start, which perhaps makes it conditional? I'm in the UK, and am looking to back out without any legal repercussions. Thank you for any help.

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