In case anyone is interested in the legalities of this, it is rather hard to explain it to non-lawyers, but I'll try. It does sound fairly technical even if you try to dumb it down, because you do have to be conversant in that type of legal language to fully understand it.
The government would be compelled to give a right of appeal against any RTB before it was enacted. The likely outcome would be multiple legal actions against the government.
The best way to get round this for any Labour Government would be to actually leave the ECHR, since such a policy is not ECHR - compliant. Furthermore, they would have to set up a different system of tribunals staffed by lets say "non-traditional" and "non-independent" judges, with no right of appeal to the courts.
What is more likely in practice, because leaving the ECHR would be a major vote loser, is that any Labour Government would realise they would lose such a case and be internationally embarrassed, so instead would raise taxes on renting. The policy would be quietly dropped.
When you consider that any case going to the ECHR on this topic would cite 400-500 other cases to be discussed, distinguished and applied, that gives you some idea of the complexity of debate. There is case law on the differential application of the concept of proportionality to the loss of control and to the deprivation of property itself and degrees inbetween. And the ECHR does not follow the strict system of precedence that there is here. Nevertheless the ECHR has strengthened the procedural aspects of the criteria of proportionality over time. There are actually another couple of cases involving Greece where Greek law was held to be illegal by the ECHR following appropriation for road building. The Court found that Greek law was “manifestly without reasonable foundation”. This made it unnecessary even to establish that the applicants had actually suffered the effects of this system, since, “in the case of a large number of owners, it necessarily upsets the fair balance between the protection of the right to property and the requirements of the general interest". I'm not giving the citation as I'm not doing work free of charge.
However, we are talking about millions of people being being forcibly deprived by the state of their property rights. Even if the tenants were only offered an option to buy, they would lose control of their properties and that in itself is sufficient for loss of property rights to be actionable under Protocol 1 Article 1.
Compensation is not adequate to redress this, because such a massive measure must be shown to be in the public interest and creating a communist utopia is not in the public interest - otherwise there would be no point in having the ECHR. Its there to protect against such things. The Greek case is distinguishable because it involved areas of forestry ancillary to the applicant's main landholding in a country whose monarchy had been disposed after a long period of political discord. It is possibly wrong on a more modern application of the law. The line of case law stemming from Sporrong and Lonroth is more likely to be followed, and the outcome almost certainly, as it nearly always is, that the aim could be achieved, on fair balance, by less onerous means - taxation. And you can also simply point out that when millions are adversely affected by a measure causing the loss of their property ownership rights, that measure is no longer in the public interest, because so many of the public are adversely affected.
In other words, its all a waste of time. Its a headline grabber by Labour and they know it, they want to attract attention by being so blatantly communist, they will appeal to those who have no hope of ever bettering their situation in life other than through taking something belonging to someone else. A sad reality.