You should look into `protect_class=` and `protection_title=` when it comes to `boundary=protected_area`. Local Purpose Reserves (Environmental and Landscape Protection) could probably be compared with UK's Local Nature Reserve if I'm not mistaken, which is `protect_class=7`. I have no idea about Local Purpose Reserve (Landscape).
[edit] Apparently the Wiki (what would have thought...) and IUCN are conflicting. Because UK LNRs are protected by national legislation, they should be IUCN Category III, IV, corresponding to `protect_class=3`. `protect_class=4`.
If you have no idea like I do, you could still add them as `boundary=protected_area` with a `protection_title`, then figure out `protect_class=` later. Many updated `leisure=nature_reserve` or added `boundary=protected_area` do not have a `protect_class=` tagged yet, or are wrong according to other datasets. The project looks very much a WIP.
>or if their insignificance means that other tags, such as landuse=grass or leisure=park are more appropriate.
As mentioned by others in your previous question, `landuse=grass` is ambiguous as to its purpose, thus should be further detailed if you so desire. `leisure=park` needs to be used as such first.