Generally speaking, an Advanced Subscription Agreement (commonly known as an ASA and designed for use in the United Kingdom), is conceptually similar to a SAFE (originally designed for use within the United States).
However, there are a few fundamental differences to be aware of, though these may vary depending on how the ASA is drafted - here are a few notable ones:
The ASA was designed to be eligible for certain tax advantages under the Enterprise Investment Scheme (EIS and the Seed Enterprise Investment Schemes (SEIS);
The ASA has a "Long Stop" term, essentially a date before which a conversion event (as defined in the ASA) must happen - otherwise the ASA automatically converts to equity on a pre-agreed price (this is not a concept in standard SAFES);
In order to qualify for the aforementioned tax advantages, it appears that the Long Stop date must be no later than 6 months after the issuance of the ASA;
6 months may not be sufficient time for a startup to reach a qualifying conversion event (i.e. a preferred equity financing) - so this is something to carefully consider.
*Please consult UK tax advice and/or counsel should you have questions about a specific set of circumstances.