Solar on a block of flats is covered by permitted development in England. Planning is usually fine. Getting the freeholder and other leaseholders to agree is the real step.
In England, permitted development covers solar PV on a block of flats. On a pitched roof the panels must stay within 0.2 metres of the slope and below the ridge; on a flat roof no more than 0.6 metres above the highest point. Listed blocks are excluded. As a leaseholder you normally need the freeholder's consent, because the roof is rarely part of your lease.