On-roof is the UK default. In-roof looks tidier but adds £1,200–£2,400 to a 4 kWp install. Most homeowners who consider it pick on-roof anyway.
On-roof installs sit on rails above the existing tile or slate surface. In-roof installs replace the tiles in the panel area entirely with a flush, flashed panel array. In-roof costs £300–£600 more per kWp installed, runs 2–4°C hotter (small output loss), but looks markedly tidier and is preferred by some planning officers in conservation areas.