Ask HN: [UK specific] is the Y Combinator SAFE or an equivalent SEIS-compatible?

I'm hoping any UK-based HNers might have experience using "advance purchase agreements" [0] in early-stage funding. In my understanding these would basically be instruments akin to the YCombinator SAFE [1], and their benefit would be that they're compatible with the Seed Enterprise Investment Scheme (SEIS).

It would seem ideal to just use the SAFE (because it's a well-established document) but of course it's not clear whether it's advisable to employ these under UK law just like that. If anyone here has used them, or a similar document that's UK-specific and that they're in a position to share, that would be awesome. I'd be super-grateful for any and all pointers and advice.

[0]: this is a term found in a letter from the Treasury, who suggest they are compatible with SEIS, but there's very little information to find about them - https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/340079/Consultation_Tax-advantaged_venture_capital_schemes.pdf (section 3.26)

[1]: https://www.ycombinator.com/documents/#safe