The simplest answer is likely to be NCO.IDE found in Part-NCO, one of the annexes to EASA Air Ops (Commission Regulation (EU) 965/2012). This annex became effective in UK in late August. Specialised Part-NCO operations, covered by NCO.SPEC, are exempt from some of the NCO.IDE equipment requirements, e.g. aerobatic flights needn't bother carrying charts, interception procedures, first-aid kits, hand-held fire extinguishers, or ELTs/PLBs. If you are using a UK C of A aircraft then the ANO schedule continues to be applicable. Schedule 4 used to be sch 5 until 1989. Aerial work may be a commercial operation but it is not commercial air transport (public transport). See an EASA FAQ
here, and some CAA guidance on Part-NCO
here.
In addition to Part-NCO and Part-SERA, which encompasses non-EASA aircraft, the UK ANO provides a legal basis for the limitations stated in the flight manual. ANO Art 33, para 7: "[a]n aircraft registered in the United Kingdom with an EASA certificate of airworthiness must not fly otherwise than in accordance with any conditions or limitations contained in its flight manual." A similar 'essential requirement' made in Annex IV of Regulation (EC) 216/2008 as amended, aka the Basic Regulation, for EASA aircraft is that "[a]n aircraft must be operated in accordance with its airworthiness documentation and all related operating procedures and limitations as expressed in its approved flight manual or equivalent documentation, as the case may be. The flight manual or equivalent documentation must be available to the crew and kept up to date for each aircraft." The flight manual limitations section may expressly require instruments beyond those in NCO.IDE/ANO sch 5; cf the flight manual subsection on "kinds of operation equipment list." Depending on the aircraft there may also be a UK CAA flight manual supplement referencing ANO equipment requirements for some operations, e.g. night/IFR, and further UK supplements may exist, e.g. a generic requirement for a warning light for alternator failures, etc.
Lastly, the certification basis specified in the type certificate data sheet for your aircraft contains lists of equipment and instruments needed for airworthiness compliance. For example a PA28-161, TC no. 2A13, falls under the US Civil Air Regulations 3 effective 15-MAY-1956, and a handful of sections in 14 CFR 23. You can access CAR 3 (15-MAY-1956) directly
here, at the FAA's Regulatory & Guidance Library (
RGL). Equipment requirements are listed in subpart F on pdf page 29 of 34. In the absence of a MEL I don't know whether the equipment listed there must be in operable condition for the aircraft to be considered airworthy; this is queried in a separate thread under this forum section.