Any co-habitee or any child of the deceased who considers a will to be unfair can make a claim under the provisions of the Inheritance (Provision for Families and Dependants) Act 1975. The claim has to be filed within 6 months of the issue of the Grant of Probate / Letters of Administration. If this is likely to happen the Executor is well-advised to limit distributions to others until the 6 months has passed. Any claim has to show that the claimant had a dependency on the deceased and purely based on what you said, it sounds to have only limited chance of success.
This 6 months period and the recent formal claim is perhaps why the uncle appears to be limiting any further payments - though he perhaps owes the beneficiaries an explanation.
It is the duty of the Executor to carry out the wishes expressed in the proven will, or potentially suffer a claim from the beneficiaries.
If it is agreed by all Beneficiaries to vary the terms of the will, the Executor can apply to the courts for a Deed of Variation. But this doesn't sound like this is agreed by all.
I can't comment on whether an individual (Uncle B) can represent themselves, but the Executor is very unlikely to want to represent the Estate himself - unless he is a solicitor himself - so what is likely to happen is that some of the estate will be gobbled up in legal expenses defending the claim.
Families, eh? It would have been so much better for this proposed will to have been discussed openly with the 5 children beforehand during the lifetime of the deceased.