Hello and thanks for using Just Answer.
I am Al and am happy to assist you with your enquiry.
I ama fraid you are not legally entitled just to change the locks, as this would potentially be seen as "illegal eviction" which is both a civil and criminal offence.
Although your Father's partner does not have any form of Tenancy agreement, she will have acquired a "Licence" to live in the property and although this does not give her full legal rights to remain living there, it does mean that a Court Order will be required before you can evict her.
I therefore recommend that you speak to a local Solicitor to advise you and to correspond with her, as regards vacating.
On a side point, if she was financially dependent on your Father, she may be entitled to make a claim against his Estate under the Inheritance (Provision for Family and Dependents) Act.
I can't of course confirm to you whether she has a claim or not, as it all comes down to the income and financial well being of each party/how long they have been together/whether she can afford to live elsewhere. However, if they had been together for many years, and she was financially dependent on your Father and has no financial means of her own to get re-housed, she MAY be successful in making a claim, to include the right to remain licving in the property for a period of time or even for the rest of her life.
This is of course only my general view, and to give you an idea of a potential outcome.
I hope this assists you and answers your question.