I'm a bit confused because it's not clear from your post who your grandfather named as the executors of his will. It's those executors who must seek probate and then distribute the estate in accordance with the terms of his will (which, from what you've written, I assume will simply mean ensuring that everything is put into your grandmother's name).
If your grandfather named a solicitor as one of the executors then (unless that solicitor has died or, very unusually, decides to relinquish the task of acting as executor) he/she must be involved in the probate process.
If your father didn't name a solicitor as one of his executors then it's entirely up to the actual executors to decide as to whether they'll take on the task themselves or seek the assistance of a solicitor. In the vast majority of cases there's no need to employ a solicitor. (I was the sole executor of my father's will and found no difficulty in obtaining probate without any legal assistance; it simply involves collecting in all the relevant information, completing the necessary forms and swearing an oath - or 'affirming' if, like me, you're an atheist).
Some points would need to be clarified though. For example if your grandparents jointly owned their house the executors would need to establish whether they owned it as 'joint tenants' or as 'tenants in common'. If they were joint tenants then your grandfather's will is completely irrelevant in relation to the house; your grandmother will automatically own the whole house now. If they were tenants in common they each owned a distinct share of the house; your grandfather's share has to pass to your grandmother under the terms of his will (which involves different Land Registry paperwork).
Either way though, the value of the house won't count as part of your grandfather's estate for the purposes of assessing whether Inheritance Tax is due. Transfers between spouses or civil partners are exempt from Inheritance Tax. (Inheritance Tax would only be relevant if your grandfather had been a tenant in common and left his share of the house to someone other than your grandmother).
So, unless a solicitor was named as an executor, I see no reason to involve a solicitor in the probate process. The executors can seek probate themselves. Once they've obtained it they might wish to consider using the services of a solicitor
solely to deal with the Land Registry forms in respect of the property but even that's not particularly difficult. If your grandparents were joint tenants then form DJP needs to be completed.
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/544454/DJP.pdf
If they were tenants in common then it's a little more complex.