Hi John
1. I very much doubt that any British Standard as specific as that you ask about exists.
2. It is likely that the solar panels are fragile, so simply relying on perimeter edge protection would probably not be an appropriate way of developing a safe system of work for maintenance access.
3. Hence to work out what is reasonably practicable, in terms of whole life cycle maintenance of the roof and whatever equipment is on it, first you need to know how often people will be up on the roof, how many at a time, and for how long and for what purposes.
4. Further to calculate the risk element of the estimation of what would be and what would NOT be reasonably practicable, you need to understand the nature of the roof - where people could fall from, to an extent how far they are liable to fall, and then the likelihood of falling at various points in the life cycle of the roof.
.......so they are more likely to fall from an edge of the roof if it has a high "pitch" (i.e. slope) and/or is of inherently slippery material and/or is wet or has e.g. moss growing on it AND if working relatively close to the edge, but probably also what direction there are typically pointing when working close to the edge BUT also how they are likely to be positioned when working close to the edge - as example someone doing felting work close to the edge of a "built up" roof is less likely to fall first because they will be kneeling and thence with a lower centre of gravity, then also they will be likely to be facing the edge and its proximity is very obvious to them, such that they are less likely to lose awareness of the hazard.
+ you need to know what part(s) of the roof are liable to be fragile. Quite probably the solar panels but also the materials for the primary construction including, obviously asbestos cement, but also e.g. RAAC and any glass or plastic rooflights amongst other weak points in the structure.
To work out how often access is needed for the solar panels, the best guidance is probably from the manufacturers or those used to installing and maintaining them.
These people can also advise on how fragile they are liable to be both at initial installation and after ageing.
+ how often they need to be replaced which affects both the frequency of access and when a solar panel should become approrpriate to consider "fragile".
5. Then I suppose the most authoritative guidance to help you consider what is reasonably practicable other than HSG33 from the HSE is the various publications from the National Federation of Roofing Contractors.
6. At which point you can consider what permanent precautions might be appropriate (having regard to other variables such as planning restrictions) and what might need to be used for each specific access. Remember that any permanent protective measures may be prone to failure - plenty of accidents have happened when permanent edge protection has failed to prevent someone falling - and these such measures may INCREASE the need for access.