Use a solicitor. Choose a fairly large firm and they will have a specialist probate department, that not only drafts wills but spends most of its time advising and acting for executors.
Avoid anyone, will writer or not, that wants to be an executor. There's big money to be claimed as a professional acting as executor. One reason why will writers can be so cheap in doing the will is that they might get to be a paid executor. They may be charging as a percentage of the value of the estate. Ordinary executors can always employ a solicitor, as and when needed, to help them, and pay for those necessary services, as they arise, out of the estate,.
No will writer needs to be legally qualified. He could be a bus driver or anyone else who fancies doing it The 'professional' ones have their own exams, but solicitors or counsel they are not. They claim to be better than solicitors because they only write wills. That does not bear examination. Yes, but solicitors write wills and act for and advise executors in administering the estates, negotiate with the Revenue and deal with every aspect from valuations and conveyancing to dealing with disputes, and claims by dependents; they know the consequences of every word in a will . The will writer is not qualified, nor does he or she have the experience, to advise or do all those things, matters in which a solicitor has had to pass exams , and then be articled, to qualify as a solicitor.