Avoid an unvented cylinder. They can go booom. ...and an annual service charge. A waste of time when superior failsafe solutions are about.
There ARE other ways, but bear in mind "they can go boom" is Drivels mantra. He is aka "watersystems" and was making similar chants. The boom was associated with early and in some cases probably diy attempts to configure a standard cylinder to unvented. If you google it, you can find one reference to a test explosion, so it was deliberate.
Unvented cylinders are constructed to meet strict requirements, and have seven safety devices including the two thermostats in the boiler. ALL of them going wrong wrong at once is unlikely.