This sounds a very difficult situation for you.
I have also directed people to Gregsons due to mentions on this site - though for property issues (buying, selling, leases), all good reports so definitely worth a call, the sooner the better I think.
For a family member's estate we used Adams & Remers (Lewes office) https://adamsandremers.com/private/
. Also caught up in an estate matter being dealt with by Turcan Connell https://www.turcanconnell.com/about-us/
(Scotland but all relevant assets in England).
Very rusty company/commercial solicitor here - I don't know the specific legal terms or procedure but there is a way to make an application, minimal cost, to register some type of caveat, notice or restriction with the Probate Registry which prevents some one else getting the grant of probate on the deceased person's estate without your notice being cleared. Gov.uk website has lots on probate/letters of administration generally.
As mentioned above - you have no right to see someone's will, it becomes a public document only once probate is granted, and the bar is high - wills only speak from death so lifetime asset changes and spending is usually irrelevant unless specific formal trusts were breached or there are mental capacity, will validity and coercion issues.
Any legal dispute involves time and cost and requires evidence, and the final outcome is either 1) an agreement between the parties involved, with maybe some form of arbitration/mediation - or 2) (worse case ultimate scenario) a court decision by a judge - much greater costs, longer time etc, and - being honest- a commercial assessment of the likely gain versus the costs, (and the stress of all of it) is worth consideration.
If you can gather all the information you have - background on timeline of events, family tree, documents, previous wills you do have copies of, (solicitors involved in drafting those) plus letters, emails, bank statements, details of the inheritances you mention, trust deeds, medical information etc etc that will all be useful to instruct solicitors.