The closes’t I have gone is 174-180mm from plane of a pitched roof, where an additional 2 extra rails from memmory were installed and the owner (a carpenter I new from site) strenghened the rafters by spreading the load of a 80’s 3 bed detached. Normally I would not go closer than 300mm which corresponds to what your installer is saying. On a hipped roof you need to pay particular attention on distance away from the sloping ridge tiles.
Its not simply a matter of additional rails, you need to know if your rafters will take the additional wind pressure, without causing stress damage to the roof or the PV module, as the uplift in the edge zone can be 3- 4 times greater than in the centre. The other issue going 200mm from the edge, risks rain overshooting the gutters.
There is a fairly simple calculation (Qp x A x Cp x SF) for wind uplift and the requirement of extra fixings and whether the rafters require any additional strengthening, which your installer should be aware of, as a member of one of the two electrical bodies (I will find something easy to explain).
Not having seen your roof, its hard to know but, yes there are possibilities of extra rails would surfice however, its how adamant your installer is and whether he would take responsibilty on MCS’s say so or not. MCS recently downgraded wind up lift and you’ll ofter find building/planning guidence is often contridictory to that of the MCS which doesn’t help.