Halifax is one of the Insurance Companies that have written their policy in such a way that if you chose not to have your dog vaccinated their whole policy can be invalidated - so that may be a consideration for you
I assume you are referring to the Halifax eXtra policy - the basic Halifax policy cover only provides £1000 a year in fees cover, wheras the eXtra covers £6000 a year, M & S is £7000 a year
there are big differences in the amount you can claim from Halifax for Behavioural intervention compared with M & S, and M & S offer twice the amount of third party liability cover than Halifax.
The accidental damage cover offered by M & S is for damage to your own property, whereas the Halifax cover specifically
excludes your own property. I've never seen the point of any accidental damage cover that excludes your own property tbh - because the third party liability element of the insurance covers damage to other peoples property anyway, so it is basically a gimmick to make cheaper policies look "as good" as more comprehensive cover
You get what you pay for - if you pay a lower premium, the cover is less; it just depends whether that element of the cover is something you feel is worth paying for or risk doing without
