Yes, you can release equity from a second property you already fully own. In the case of a holiday home, there will be conditions that it must be available for the sole use of the owner, or only let out for a maximum of 4 weeks consecutively. It must not be advertised as a holiday let anywhere. Also, to qualify as a second home in the eyes of the provider, you must spend at least 4 weeks of the year there, and it must not be in close proximity to your main residence.
With buy-to-let properties, it is also possible, and certain providers have set up dedicated buy-to-let Equity Release schemes to allow property owners to obtain capital to help grow their property portfolio, as well as give them an extra income.