Critical illness is much more expensive than life cover. Your broker makes more commission from the life assurance than he does from the mortgage. I used to get 140% of annual premium as commission when I sold it. (£1344 on a £80 per month policy). This cost has to be covered somehow.
Egg do life cover without commission so it is cheap. But, I got mine with L&G through a broker. He made commission, but I appreciated his help - lots of paperwork if you do it yourself.
I have £100K CIC on myself (29 yr old, non-smoker), and £200K joint life with my wife (27 yr old, non-smoker).
If I get diagnosed with say cancer, I get immediate payout of £100k, whether I die or not. If I do die, the £200k pays out.
I pay £40 per month for this cover.
Critical Illness is probably not mandatory for the purposes of covering your mortgage.