It is searches like this that Family history Societies are very good for. No matter where in the UK your maternal grandmother was buried, there will be a local family history society branch covering that area, Family History Societies are registered charities, and they retain their charity status by various projects to record and publish the Monumental Inscriptions of cemetaries and graveyards.
In the first instance I suggest you write to the secretary of the society that covers the area you find your grandma died in, and give them as much info as possible taken from the death certificate. they will be able to check their recorded inscriptions and if they do not find anything, they could then offer the services of one of their local researchers to perhaps track down the grave in the cenetaries or graveyards they have not recorded.
It would also be possible to check burial registers in the local churches that were performing burials at the time she died, this is one way of eliminating possibe burial sites. This can be done by post once you have the info from the GRO death certificate. It is probaqbly not going to be a quick search but it will eventually be successful i would guess.
This is the addy for the federation of family history societies:
http://www.ffhs.org.uk/
you will find the contact details for the county societies on there.