Thanks for your enquiry and am sorry to hear of your loss.
You will indeed be entitled to enter into a Deed of Variation, which will need to be signed by you and your brother at any time up to 2 years from the date your Father passed. It would be normal for the Deed to be prepared after Probate has been granted but there is no harm in entering into it before Probate has been granted. A local Solicitor is likely to charge you £250-£350 plus VAT for preparing the Deed.
I hope this confirms the position, but please let me know if you require any clarification.