I had my first will done, by a will writing company (this choice was against my better judgement but I was over-ruled). To be fair, what was done, and the fee, was OK for what it was, but it was clear fairly quickly that I (totally a lay person who'd once read a book) knew more about wills than the will writing person. Though in fairness
it was OK there is no way they'd cope with anything not 100% simple.
Next one (changed circumstances, not entirely straightforward as pending a divorcez), was done by a local solicitor. It was reasonably priced, and I got emailed a copy in word, which was helpful for later additions sans solicitor. The guy subsequently did my flat purchase and I thought it telling that he gave me a more detailed and accurate assessment than the so called surveyor despite the lawyer never setting foot in the place, though perhaps that's a moan for a different day.
Anyhow, if will is "straightforward" eg "all to mrs and kids" then you can just buy a book and follow what the rules are. If there are trusts, tax management, ex-spouse, or likely soon to be ex-spouse or new spouse, then you need a solicitor to avoid Mr Cock up. A will writing service will not cope with any of the above.
So, here's my advice
1 Buy and read a current UK law (and Scottish law if applicable) wills and probate book eg which or similar
2 Write up a list of who is to get what
3 If "straightforward" write it all up in a will
3 (b) if not "straightforward", get a lawyer to write it all up from your draft. As with any tradesman, I'd get a recommendation if possible.
4 Choose executor(s) plus backup(s) - probably the primary beneficiary (ies). I would not appoint a solicitor as when
the time comes the family solicitor in question is likely dead, retired or bought out by PWC and will charge you a mint. A spouse or adult child executor can hire a lawyer of their choosing and claim the fee from the estate if anything tricky needs doing or it just gets too much, but the control remains with them.
4 Get your signining witnessed by two people who do not benefit IN ANY WAY, from the will ,not even a teacup, else whole will is invalid.
Anyhow, the "rules" aren't that hard, read up upon and avoid the pitfalls.
One thing to consider, particularly for a childless couple is what happens if you both die in the same accident. And if you have small children, same question but even more so.