You really need to speak to an advisor to understand the products better. If you make a quote on something like moneysupermarket, they'll call you and explain it to you. This is free, and is just about making sure you understand what you're potentially buying.
Income Protection and Terminal Illness have different purposes. The intention of Critical Illness cover is to cover the costs you incur with a serious illness, which may not be terminal or even stop you from working. Things like going to appointments on the other side of the country, a spouse having to take time off work to help during your treatment, adjustments you make to your home etc. It's worth noting that some Critical Illness policies also include a sum for dependent children - again to cover things like time off work to care for them. There's quite a big difference in which conditions are covered in different products so make sure you understand that. You pay more for more substantial cover, obviously.
Income Protection pays out if you can't work. Check what is required to pay out: it may be being unable to do your own job, or it may be being unable to do any job/do normal day-to-day things. Another thing to be aware of is that you can normally choose how long your deferred period is, ie how many weeks you need to be unable to work before it pays out. If you have enough personal savings to cover the first 3 months or 6 months of being unable to work (ie you're protecting yourself only for long term inability to work) then your premiums will be lower.
If you only have descending Life Cover, does that mean it's only to pay off the mortgage, and you each earn enough to be OK for living costs if the other dies? That can be the right solution if you have 2 equal earning partners with no dependant children, but it's still a bit strange to have 2 separate descending life policies.
It's common to have one descending joint life policy for you and your spouse jointly to cover the mortgage and then additionally each have a separate single life Life policy - usually either level or increasing - to cover living costs for any dependants.
The joint life policy only pays out once when the first one of you dies - which is appropriate for paying off a mortgage since you only need to pay it off once even if you both die (2x single life policies is more expensive since if you both die it will pay out for each of you) - and it's descending to reflect the reducing debt of a repayment mortgage.
The single life policies cover a percentage of the salary you each earn, to cover living costs for dependants. These can be for different amounts if you earn different amounts, but don't underestimate what it would cost a surviving spouse to pay for someone to provide the childcare etc which a SAHP/part-time worker contributes to the family. It's often appropriate to have separate policies for this since each of you contributes a certain amount, and if you both died then whoever looks after your dependants may need both lots of cover to take over their care.
Finally, check whether your Life Cover includes Critical illness. Some products cover both - which is obviously more expensive than Life Cover only, but you don't want to have double the cover you need without realising. Some Life policies may also offer 'Accelerated' cover, which means that in specific circumstances, eg if you have an illness which is expected to be terminal within a short time, then they'll pay out whilst you're still alive, so that you can use it during the expensive end-of-life period.
The main thing is, be really clear about what risk it is you want to cover, and then make sure you've got the right cover for it. An advisor can talk it through with you and help you make sure of that.