Sorry if the guide is not clear – I’ll have a poke around and see if I can improve the language. Opt out is on the basis that it is practical and reasonable. For many large estates this not be possible for infrastructure reasons but what IS possible, it binning the whole proposed scheme and going for something that all (ok, the majority of) tenants and leaseholders are happy with.
To put it another way, an estate can opt out of communal central heating and opt for individual boilers. This would depend on infrastructure, but given the proposed costs of some of the schemes, even paying for that infrastructure is worthy of detailed consideration.
The Forum has now (just!) got a copy of the latest Heating Optional Appraisal and this clearly states that individual boilers should be considered: http://democracy.camden.gov.uk/documents/b16759/Appendix%20A%2028th-Jul-2015%2018.45%20Leaseholders%20Forum.pdf?T=9 / “Replacement of inefficient systems with individual or communal heating whichever provides best overall value for money and secures the required carbon emission reductions” to be considered.