Whenever someone dies (irrespective of whether they leave a will or not), the people who need to access their assets (in order to distribute them in accordance with the terms of the will, or under the intestacy rules) require a 'grant of representation' from a court. If there's a will, such a grant is in the form of a 'grant of probate'; if the person dies intestate, the grant is in the form of 'letters of administration'.
The only exception is where the value of the estate is less than £5000 (not £25,000, as referred to in the question). Under such circumstances the law does not require a grant of representation to be obtained (but it might still be necessary anyway, as some financial institutions will not release the assets of a deceased customer without a grant).
When your mother passes away it won't be good enough to simply produce her death certificate (and her will) to the building society, in order to access her assets (even if the will names you as both the sole executor and sole beneficiary of her estate). Her executor(s) will need to convince the Probate Court that they have produced her final will, that they've established the full extent of her assets and debts, and formally swear (or 'affirm') that they will act in accordance with the terms of the will. Only then can the necessary grant be obtained. (Her will, and details of her estate, will then pass into the public domain).
Chris