How strict this policy is varies a little from one bank to the next but it's unlikely that you will be able to get this condition moved to the loan offer. The whole point of having probation completed is to give both you and the bank the comfort of you being permanent and past probation in your new job before you commit to signing contracts and paying a non-refundable deposit on a house. If your probation were to be extended, the bank would not release the funds and you would risk losing your deposit. If the lender will not change it to being a pre-drawdown condition, then you would either have to push out the date for signing contracts, or get your solicitor to include a clause stating that your deposit will be refundable if you cannot obtain a mortgage.
Best regards,
Dave Curry (broker)
https://www.linkedin.com/in/davecurryirl