Decreasing Term Assurance is really only appropriate when used in conjunction with a repayment mortgage. The payout decreases over the years as the amoun of your mortgage loan decreases. You take out the policy for the same term of the mortgage and starting with a sum assured the same as the loan amount.
Level Term Assurance can be for any amount and for any term and for any reason and is commonly used for family protection. The most common method of working out how long the policy runs for is either long enough of children to have finished further education or your expected retirement age. The sum assured is your choice but if it is for family protection a good rule of thumb would be between 5 and 10 times your salary/income. The amount of the payout doesn't change from day one to the end of the term, though if you survive to the end of the term, you get nothing back.
If you are concerned about the cost, a variation on term assurance is 'family income benefit'. This works in a similar way in that it pays out if you die, however, instead of paying out a lump sum, it pays out an income for the rest of the term of the policy. Again, you set the sum assured (income level) which is paid out tax free. The amount of the total potential payout reduces as the remaining term of the policy reduces and consequently is cheaper than level term assurance.
Whole of life insurance can be more expensive but not always, however, the premiums tend not to be gauranteed, unlike term assurance, and are reviewed after 10 years and then every 5, so can increase to a level that is no longer affordable. The plus side being that there is no expiry date as long as you keep paying the premiums.
Any type of life cover will be subject to underwriting by the policy provider which will take in to account health and lifestyle.
If the policy is just on your life, make sure it is written in trust or on 'life of another' basis where your hubbie is the policy owner. That way, the payout would be quicker and would not become part of your estate.
As has been commented above, you should really get advice from an IFA (not the bank).