You need to remove the white residue otherwise no treatment will hide these. Pencil eraser or peanut oil or a good scrub with a household all purpose cleaner.
as for treatment you won't find much worth the money in halfords, most will wash off in the morning dew, especially the autoglym offerings, look nice until the weather forecast predicts rain and it'll be washing off before you even know it's raining.
a decent dressing like autofinesse revive, carpro Perl, chemical guys new look trim dresssing are worth waiting a few days for delivery, spend this time on cleaning and prepping