I have a lot of experiance in instructing solicitors through my line of work (Financing property development). As with everything you get what you pay for however with the legal professionals their is a huge range.
Typically the domestic house selling and buying conveyencing type work is the bottom of the pile. Its 90% donkey work that can be done by anyone anywhere and lots gets outsourced to low rent service providers. This is they type of stuff that gives firms bad names as it takes a long time and little errors cause long delays. If you use a local firm you can still get decent service if the partner/lawyer you are dealing with is not bogged down with this rubbish.
Then there is mid level firms who have lots of staff and dont get involved with domestic conveyencing. They do the more involved work and do it very well. But will charge a bit more.
And you have the massive corporates that do a bit of the mid level stuff but they exist to work for other big corporates and have massive liability insurance and will charge massive fees.
One other point. It is your MIL who would engage a solicitor for her will etc – you should stay at arms length to avoid any issues. Then when she pops off you can instruct a soliocitor to deal with the estate assuming you and the other executors agree how to proceed. They need not be the same firm, everything is recorded centrally now so the actual paper work and which firm holds it should not be an issue.