You could consider setting up an appointment for your uncle's brother to see his own solicitor for independent legal advice, having had your solicitor send them the consent form in advance and a copy of your application, and paying for it. That way the other solicitor will be able to assess whether or not he does have a valid ground of objection and advise him accordingly. If he doesn't have a valid ground and really does want to allow your application to proceed maybe then he will sign the consent form.
Alternatively do your own affadavit to the land registry, setting out that you have spoken to him twice, that he has verbally informed you that he no longer wishes to oppose your application but that to date he has refused to sign a consent, that he does not, in any case, have a valid ground for objection and ask once again for registration to proceed. You could ask the land registry to send another notice to him- while he may not want to sign a consent form, he may simply not reply to a further notice which would then allow the land registry to go ahead.
The day the application is complete you can sign over a site to your daughter.