They are all as bad as each other in reality, the only thing you need to consider when deciding which to use/trust is which offers the best compensation in case it goes missing.
A reason some are cheaper is they offer limited compensation, when you start paying for the extra cover it no longer becomes the cheaper option, also Royal Mail loose a lot more parcels/letters than any other carrier, just something to think on and to balance it out. The last normal year, before the current mess, they paid out over £4M in compensation for lost parcels. In 2020 they acknowledged over 15M letters, yes Million, were lost