It's unfortunate that your father chose to name a solicitor (or a solicitors' practice) as an executor. He was clearly ill-advised in the matter (quite possibly by the relevant firm of solicitors).
Now that it's happened though, the only way to remove the solicitors' practice as an executor is through a court order which, unless the solicitors' firm agreed to the court application (which I would regard as highly unlikely), would require you to produce evidence of malpractice (or another clear reason as to why such a change should be made).
Sorry!
https://www.holmes-hills.co.uk/news/2022/january/can-you-remove-an-executor-from-their-role/