Under the Consumer Credit Act a borrower can waive life cover if the property being mortgaged is not their main residence - which is the case with your mother - however lenders will look at this on a case by case basis. Which lenders have you aproached so far?
Also under the CCA, life cover can be waived if
1. you (or your Mum) are over 50
2. Declined cover
3. subjected to a severe loading on life over (must be so much that the policy is not affordable - usually 150% of normal price)
Has anyone been refused a mortgage because they could not get life cover?, a friend of mine is struggling to get life assurance because of a previous illness and has mortgage approval to buy an apartment which is supposed to close in the next few months.