UK residents with indefinite leave to remain aren’t taxed by the UK? Strange assertion.
A UK citizen will not be taxed if he renounces his citizenship. With the ease of immigrating to the EU, proximity of Europe, and the existence︀ of CTA, there is little reason to keep UK citizenship in that case. It’s a︁ different proposition than the situation for US citizens. So, yes, it is ultimately about immigration︂ (or emigration, depending on perspective).