For my current conversion I have used Sikaflex EBT to fix the solar panel although I always use mechanical fixings as well being a disbelieving type.
Sikaflex EBT is aimed at the building trade but the fact sheets from Sika indicate that it is suitable for roof tiles, shop fittings etc. so I thought I would give it a try. Fact sheets are available at https://www.sealantsandtoolsdirect....eige_brown_black_grey_white_clear_P26598.html . Screwfix sell it at £7.39 for a 300ml cartridge, colours available are black, white, beige, brown, clear and grey although I don't think Screwfix have all colours.