Guidance on form, grammar, puntuation, and usage from various bodies in the United Kingdom.
The European Commission's English Style Guide contains a general-purpose writing section before covering technical detail specific to the European Commission and Union.
The GDS maintains web-based guidelines for content published on GOV.UK.
Grammarly requires you to use an account to access its service, even the free version. There are no limitations on the use of the free version, but it does not have the advanced tonal and rewriting features of the premium tiers.
Hemingway is focused on simple improvements, including using the active voice and easier readability. It is a free service to use online, with a paid downloadable app.
The MOD provides guidance for defence writing in Joint Service Publication 101.
This Microsoft Word template fits the majority of JSP 101 requirements.
The Development, Concepts and Doctrine Centre within the MOD also publishes this writer's guide.
Standards for written communication from the ONS are contained in their Style.ONS minisite.
The free version of ProWritingAid is limited to 500 words at a time and doesn't provide the full range of stylistic suggestions.