Hi Gary and thanks for using Just Answer.
My name is ***** ***** am happy to assist you with your enquiry.
I am afraid he will certianly need Mr B's consent/agreement, as he has an equal entitlement under the overage/uplift agreement. If you were to pay off Mr A alone, Mr B could still enforce the uplift provision, so you would be in a worse position than you are now!
On a side point, it would be normal for their interest in the property to be registered as a Restriction on your title. In order to remove such a Restriction, Mr A and B would need to sign the appropriate Land Registry form withdrawing this Restriction from your title.
I hope this helps and sets out the legal position.