In a democracy an individual has an entitlement to cast a vote. On the other hand, is it possible that the entitlement to vote is, at the same time, a responsibility? An entitlement is a thing you are fortunate to be permitted to do. Preventing Labour party members from voting on an issue is infringing on their rights.
It is prohibitively exclusive at best and unethical at worst to prevent newer members of a political party from voting on who should lead the party in question. England is a constitutional democracy, meaning much political power is in the hands of the Parliament. Voter suppression is rarely, if ever, fair. Also, many of these people paid a fee to become a member of the Labour Party, so it would be fraudulent for the Labour Party to accept their money and then not give them the full privileges of the party.
The whole point of democracy is to allow everybody the chance to vote on the things they care about. It does not give you the right to pick and choose who gets to vote on certain things. Once you start doing that then you start going into fascist territory and nobody wants that. Let people vote how they want and let the chips fall where they may.
Everyone should be able to vote, regardless of party affilation. Elections are not fair and it is not a true democracy if these kinds of rules are broken by the people who are in charge. The country is no longer a democracy when elections start to become rigged like this.